4 medium zucchini, sliced crosswise into 1/4” coins
2 tsp. freshly chopped thyme
Freshly chopped parsley, for garnish
Preheat oven to 375° and butter a medium casserole dish. In a large skillet over medium heat, melt butter. Add garlic and cook until fragrant, about 1 minute. Whisk in flour and cook until flour is golden and starts to bubble, about 1 minute more. Add milk and stir until mixture comes to a simmer. Boil until slightly thickened, about 1 minute.
Turn off heat and add 1 cup Gruyère and Parmesan. Stir until cheese has melted, then season with salt, pepper and nutmeg.
Add a layer of zucchini to the baking dish, overlapping the zucchini slices. Season with salt and pepper and pour about one-third of the cream mixture over zucchini. Sprinkle some of the remaining Gruyère on top, then sprinkle thyme on top of cheese.
Make two more layers with remaining zucchini slices, cream mixture, cheese and thyme. Bake until bubbly and golden on top, 23 to 25 minutes.