Collard Green Gratin

INGREDIENTS

• 1 cup finely grated Parmesan, divided
• Kosher salt, freshly ground pepper
• 1 large onion, thinly sliced
• 4 tablespoons olive oil, divided
• 1/4 teaspoon freshly grated nutmeg
• 1 cup coarse fresh breadcrumbs
• 2 garlic cloves, finely chopped
• 2 tablespoons all-purpose flour
• 2 tablespoons unsalted butter
• 4 ounces very thinly sliced country ham or prosciutto
• 1 teaspoon chopped fresh thyme
• 2 cups whole milk
• 2 bunches collard greens (about 1 pound), center ribs and stems removed