Zucchini Gratin (French Gratin de Courgettes)

Steps

1. Preheat the oven to 425°F (220°C). Slice the zucchini into 1/4-inch (5 mm) rounds. On 2 parchment-lined sheet pans, toss them with the olive oil and 1 teaspoon of salt, then spread them in a single layer without overlapping: crowded, they would steam.
2. Roast for 20 to 25 minutes, swapping the pans halfway through, until the rounds have shrunk by about 40%, are tender and golden at the edges. This is the step that prevents a watery gratin, so don't cut it short. Take them out and lower the oven to 375°F (190°C).
3. While the zucchini roasts, melt 15 g (1 tablespoon) of the butter in a skillet over medium heat. Add the thinly sliced onion and cook for 8 minutes, stirring, until soft and golden. Add the minced garlic and the leaves stripped from the thyme and stir for 1 minute.
4. In a large bowl, beat the eggs with the cream, milk, 1/2 teaspoon of salt, the black pepper and the grated nutmeg. Stir in 100 g (1 cup) of the Emmental.
5. Butter an 8 x 12-inch (20 x 30 cm) baking dish with the remaining 5 g of butter. Gently toss the roasted zucchini with the thyme onions, then arrange them in the dish in even layers.
6. Pour the custard over the zucchini: it should come just below the top layer. Tap the dish on the counter so it seeps in everywhere, then scatter the remaining 100 g (1 cup) of Emmental over the whole surface.
7. Bake for 30 to 35 minutes at 375°F (190°C), until the top is deep golden, the edges are bubbling, and the center no longer wobbles when you shake the dish. A knife pushed into the middle should come out with no liquid custard.
8. Let the zucchini gratin rest for 10 minutes out of the oven before serving: the custard finishes setting and the portions hold their shape when cut.