Adapted from a recipe by Domenica Marchetti
Layered strips of grilled zucchini with mozzarella and parmesan cheeses and a basil-infused béchamel.
Lightly coat zucchini slices with olive oil on one side (preferably the cut side); season with salt and black pepper. Working in batches, grill zucchini on the rack of a covered grill over high heat on the coated side only for 4 to 8 minutes or until char marks form; remove. Set aside.
Preheat oven to 375°F. Butter a 13" by 9" rectangular baking dish.
To make the white sauce, in a medium saucepan melt butter and cook onion over medium heat for about 5 minutes or until softened. Add the flour; cook 3 minutes or until light golden brown, stirring constantly. Add the milk, whisking to combine well; bring to a boil. Reduce heat; simmer for 2 minutes, stirring often. Stir in basil, nutmeg, 1/2 teaspoon salt, and 1/4 teaspoon freshly ground black pepper. Remove from heat.
To assemble the casserole, start off by spreading a third (about 2/3 cup) of the sauce on the bottom of the prepared baking dish. Next, add a layer of zucchini strips with the grilled sides facing up. Then sprinkle half of the shredded mozzarella and half of the parmesan over the entire surface. Repeat with a layer of sauce (it will be quite thick so just apply small dollops evenly over entire surface) then zucchini strips followed by the remaining mozzarella and parmesan. Finish it off with the rest of the white sauce then the remaining zucchini strips placed so that the top layer features alternating grill marks.
Bake, uncovered, for 40 minutes or until bubbling and nicely browned. Remove from the oven; let stand for 20 minutes. Top with additional fresh basil and flaky sea salt.