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This Garlicky, Herby Summer Gratin Uses Four Pounds of Vegetables

Zucchini, summer squash, and tomatoes shine in a fresh French casserole.
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Published Aug. 8, 2023.

I have a small but productive garden that’s just steps away from my back door. It’s surrounded by a picket fence, and over the years, a climbing hydrangea has grown over the arched entry, framing it with big white blooms. Aside from the kitchen, it’s my favorite place to be.

Come late July, when all goes well (hungry bunnies, keep out!), the raised beds start to overflow with zucchini, summer squash, tomatoes, onions, and herbs, and I know exactly what to cook: a Provençal-style casserole that calls for 4 pounds of produce.

It’s been 15 years since I developed my Summer Vegetable Gratin, and it’s proven to be timeless. The sweet-tart tomatoes and juicy squash concentrate in the heat of the oven, and a layer of caramelized onionsalong with a golden brown, crispy-cheesy toppingprovide rich contrast to the bright, tender vegetables. Basil, thyme, and garlic feature prominently, too, ensuring that every bite is full of flavor.

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Full disclosure: This gratin takes a bit of time to prepare, but none of the steps is difficult and the result is so worth it.

Keys to an Intensely Flavorful Summer Vegetable Gratin

A few simple tricks make the vegetables sing.

  • Get Rid of Excess Water: Peak-season produce is super-juicy. To avoid a watery gratin, salt slices of zucchini, summer squash, and tomatoes and let them stand for about 30 minutes before blotting away excess moisture with paper towels. (This step ensures that the vegetables are thoroughly seasoned too.)


  • Add a Layer of Caramelized Onions: Slowly cook two sliced onions in olive oil until they are softened and deep golden brown to add rich, sweet-savory depth to the bright vegetables. (This step can be done in advance, if desired.)


  • Coat the Veggies in Herby, Garlicky Oil: Blend fresh thyme, minced garlic, black pepper, and extra-virgin olive oil together and drizzle it all over the vegetables to ensure that every bite is packed with flavor.


  • Season the Topping Too: Top the casserole with a mixture of fresh bread crumbs, finely chopped shallot, Parmesan cheese, and a little olive oil. Garnish the finished gratin with lots of chopped fresh basil.
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Vegetables Illustrated

This is the book for anyone looking for fresh, modern ways to add more vegetables to their diet—which is just about all of us. And this exciting guide could have come from only Cook’s Illustrated.

The gratin makes a wonderful side to simple grilled fish or chicken. I also like it with lots of crusty bread (and perhaps a few nice French cheeses) as a light, high-summer main course. 

Ready to cook? Click here for my full recipe for Summer Vegetable Gratin. Or, if youre up for something a little different, consider my Summer Vegetable Gratin with Roasted Peppers and Smoked Mozzarella.


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