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The Cheesiest Zucchini Gratin in the World

The Cheesiest Zucchini Gratin in the World

Thinly sliced zucchini is layered with a creamy Gruyere cheese sauce and finished with Parmesan breadcrumbs. Salting and drying the squash first keeps the gratin rich and bubbling instead of watery.
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Prep Time 20 minutes
Cook Time 35 minutes
Total Time 1 minute
Servings: 4 servings
Ingredients Method Notes

Ingredients
  

Zucchini and Sauce
  • 2 pounds zucchini or summer squash, sliced into approximately 1/4-inch rounds
  • 1 teaspoon kosher salt, plus more for seasoning
  • 2 tablespoons unsalted butter
  • 1 small yellow onion, finely chopped
  • 2 garlic cloves, minced
  • 2 tablespoons all-purpose flour
  • 1 cup whole milk
  • 1/2 cup heavy cream
  • 1/2 teaspoon freshly ground black pepper
  • 1/4 teaspoon grated nutmeg, optional
Cheese and Topping
  • 1 cup Gruyere cheese, grated
  • 1/2 cup finely grated Parmesan cheese, divided
  • 1/2 cup panko or fresh breadcrumbs
  • 1 tablespoon olive oil or melted butter
  • 1 tablespoon fresh or 1 teaspoon dried fresh thyme, chopped, or dried thyme

Method
 

Step-by-Step
  1. Heat the oven to 375°F (190°C) and butter a 2-quart gratin dish or shallow baking dish.
  2. Toss the sliced zucchini with 1 teaspoon kosher salt and let it stand for 20–30 minutes. Pat the rounds thoroughly dry with paper towels.
  3. Melt the butter in a saucepan over medium heat. Cook the onion until soft, about 5 minutes, then add the garlic and thyme and cook for another 30 seconds.
  4. Stir in the flour and cook for about 1 minute. Gradually whisk in the milk and heavy cream, then simmer for 3–5 minutes until lightly thickened; season with black pepper and optional nutmeg.
  5. Remove the sauce from the heat and stir in the Gruyere and about 1/4 cup of the Parmesan until smooth and melted.
  6. Arrange half of the zucchini in the prepared dish. Spoon over half of the cheese sauce, then repeat with the remaining zucchini and sauce.
  7. Mix the breadcrumbs with the remaining Parmesan and the olive oil or melted butter. Scatter this mixture evenly over the gratin.
  8. Bake for approximately 35–45 minutes, until the zucchini is tender and the sauce is bubbling. Broil for 1–3 minutes at the end if you want a deeper golden crust, then rest for 10 minutes before serving.

Notes

Salt and dry the zucchini thoroughly to prevent excess liquid. Use a shallow dish and avoid stacking the slices too thickly so the gratin cooks and browns evenly.