cauliflower gratin

INGREDIENTS

• 1 (3-pound) head cauliflower, cut into large florets
• Kosher salt
• 4 tablespoons (1/2 stick) unsalted butter, divided
• 3 tablespoons all-purpose flour
• 2 cups hot milk
• 1/2 teaspoon freshly ground black pepper
• 1/4 teaspoon grated nutmeg
• 3/4 cup freshly grated Gruyere, divided
• 1/2 cup freshly grated Parmesan
• 1/2 cup fresh bread crumbs