Cheesy Zucchini Gratin with Homemade Breadcrumbs

This decadent zucchini gratin turns a mountain of summer squash into a golden, bubbling side dish with crispy breadcrumbs on top and a gooey béchamel sauce underneath. The zucchini gets a quick head start in a skillet, then everything bakes together in a single gratin dish, which works just as well for a weeknight dinner as it does for a holiday table.

A gratin is a classic French recipe, and the technique couldn’t be simpler: layer your vegetables, cover them with cheese and breadcrumbs, and let the oven do the rest. So when summer squash starts piling up at the market (and taking over my garden!), this gratin is my favorite way to turn it into a side dish everyone loves.

If your garden is overflowing with summer squash, put it to good use in my Easy Zucchini Bread, Chicken Brochettes with Zucchini Salad, and Provençal Tian!

Why You’ll Love This:

• You’ll love the contrast of textures. The tender rounds of zucchini are tossed with a silky béchamel sauce, a layer of melted Gruyère, and delicious crunchy homemade breadcrumbs. Every bite is gooey, creamy, and crunchy all at once!
• It’s also a forgiving dish when entertaining. You can bake the whole thing the day before, then simply reheat it when you’re ready to serve.
• It’s substantial enough to serve as a vegetarian main course, but simple enough to serve as a side.
• It pairs beautifully with my Roast Chicken recipe or a holiday beef roast.

Ingredients:

• Nutmeg: Just a pinch, to give the béchamel sauce a nice nutty flavor.
• Zucchini: Sliced into 1/8″ rounds, this is a wonderful way to put an end-of-summer zucchini harvest to good use.
• White Onion: Sliced into half-moons 1/4″ thick and sautéed with the zucchini, it lends a gentle sweetness that balances the richness of the cheese and sauce. A yellow onion works just as well.
• Olive Oil: For sautéing the onion and zucchini in a large skillet, which takes the raw edge off the squash before it goes into the baking dish.
• Gruyère Cheese: This gives the gratin its gooey, nutty melt.
• Sourdough Bread: Sourdough bread works best for making these breadcrumbs, as its sturdy texture holds up to the blade of a food processor, and its subtle nooks and crannies make a nice vehicle for holding the melted butter!
• Butter: Does triple duty, greasing the baking dish, binding the bread crumbs, and forming the base of the béchamel sauce.
• Garlic and Fresh Parsley: Minced into the breadcrumbs for that garlicky, herby crunch against the creamy sauce.
• Flour: Whisked into the melted butter to make the roux that thickens the béchamel.
• Milk: The body of the béchamel sauce. I use whole milk for the silkiest result.

How Do You Slice Zucchini for a Gratin?

This recipe calls for 7 cups of zucchini sliced into 1/8″ rounds. You certainly can slice them by hand, but I find a mandoline does much quicker work and gives you uniform rounds that help the zucchini cook evenly. I use mine a lot in the summertime for things like my Creamy Cucumber Dill Salad, and in the fall for my Brussels Sprouts Salad. If you cook many recipes that call for evenly sliced vegetables, a mandoline is a worthwhile investment.

Step 1: Sauté the Zucchini and Onion

• Preheat your oven to 375°F (190°C) and grease a 9″x13″ gratin pan or baking dish with butter.
• Heat the olive oil in a large, deep skillet. Add the onion and sauté until soft and translucent, then season with salt and pepper.
• Add the zucchini and sauté for 3 to 5 minutes.
• Cover the skillet and let the zucchini steam for about 10 minutes, shaking the pan occasionally to toss the vegetables, until they’ve softened and turned slightly transparent.

Step 2: Make the Homemade Bread Crumbs

• The homemade breadcrumbs are what really make this gratin sing.
• Tear the sourdough into pieces and add them to a food processor along with the melted butter, minced garlic, fresh parsley, and a little salt and pepper.
• Pulse until you have a coarse crumb, then set aside.

Step 3: Make the Bechamel Sauce

• Béchamel is a classic French sauce, and it’s easier to master than you might think. Once you have it down, you can use it for all sorts of things, like my Easy Baked Mac and Cheese, Chicken and Mushroom Crepes, or my Creamy Sundried Tomato Pasta.
• Melt the butter in a large skillet, then whisk in the flour and let it cook for 1 minute.
• Slowly pour in the milk, whisking all the while, until the sauce is smooth.
• Add the nutmeg, salt, and pepper, then simmer until the sauce thickens and coats the back of a wooden spoon.
• It should look like a thick Alfredo sauce. I take it a bit further in the thickening stage than I normally would because zucchini is watery and will release its juices, thinning the sauce as it bakes. Don’t be afraid to have it on the thick side.
• Then add the zucchini/onion mixture and toss to coat.

Why This Gratin Gets a Head Start

Since the gratin only needs about 30 minutes in the oven—just enough to melt the Gruyère and toast the breadcrumbs—the zucchini needs a head start on the stovetop first. Without it, the zucchini would still be a bit raw and crunchy by the time the top turns golden. That two-step method of sautéing and steaming the zucchini before it goes into the baking dish is what gives you the best texture.

Step 4: Top and Bake

• Transfer the mixture to a gratin pan.
• Scatter the freshly grated Gruyère over the top, then sprinkle with homemade breadcrumbs.
• Bake at 375°F (190°C) for 30 minutes, until the sauce is bubbling and the bread crumbs are golden brown.
• Serve oven-to-table.

Substitutions and Variations

• Use yellow squash in place of some of the zucchini, or a mix of both summer squash, for a gratin with a bit more color.
• Stir a little shredded rotisserie chicken into the béchamel sauce, and this gratin becomes the main meal rather than the side.
• Any rustic loaf will work for homemade breadcrumbs; a boule is a great alternative to sourdough. You want a bread sturdy enough to withstand the blade of a food processor, with plenty of nooks and crannies to hold the melted butter.

Do You Have to Salt the Zucchini First?

No. A lot of zucchini recipes have you salt and drain the rounds to draw out moisture, but I skip that step here. Sautéing and then steaming the zucchini in a large skillet first cooks out the excess water, so the gratin bakes up silky rather than watery without any salting.

Do You Need to Peel the Zucchini?

There’s no need to peel it. I slice the zucchini into 1/8″ rounds with the skin left on, which is exactly how it goes into the skillet before baking it.

Can You Make Zucchini Gratin Ahead of Time?

Yes, this gratin reheats beautifully. Complete all the steps and bake it, then let it cool completely, cover, and refrigerate. When you’re ready to serve, place it in a 350°F (175 °C) oven, covered with foil, for 20 minutes. Then remove the foil and bake for another 7 to 10 minutes, until the bread crumbs have crisped back up and the sauce is bubbling.

What Can I Serve with Zucchini Gratin?

This zucchini gratin is a terrific side dish for my Roast Chicken in a Skillet, and it’s just as good alongside my Coq au Vin Blanc (White Wine Chicken) when you want something a little more French for the table. It’s also filling enough to stand on its own as a vegetarian main course, which makes it a nice meatless option for the Thanksgiving table.

Pro Tips

• Run the zucchini through a mandoline for even 1/8″ rounds, which makes quick work of 7 cups and keeps the slices cooking at the same pace.
• Be sure to give the zucchini a head start on the stovetop, sautéing and then steaming until the rounds are soft and slightly translucent, so they aren’t still raw when the top is golden.
• Don’t over-process the bread crumbs. A few pulses will do the trick and keep them light and crunchy.
• Grate the Gruyère fresh for the smoothest melt.
• Bake it and reheat it later if you’re entertaining, so the gratin is out of your way before guests arrive.