Zucchini Gratin

INGREDIENTS

• 1/4 teaspoon ground nutmeg
• 1 cup hot milk
• 2 pounds zucchini, sliced 1/4-inch thick (4 zucchini)
• 6 tablespoons (3/4 stick) unsalted butter, plus extra for topping
• 3/4 cup fresh bread crumbs
• 2 tablespoons all-purpose flour
• 1 pound yellow onions, cut in 1/2 and sliced (3 large)
• 1 teaspoon freshly ground black pepper
• 3/4 cup grated Gruyere
• 2 teaspoons kosher salt