Mashed Potato and Cauliflower Gratin

INGREDIENTS

• 2 pounds russet (baking) potatoes, peeled and cut into 1-inch pieces
• Salt
• 1 1/4 cups whole milk, divided
• 1 stick (8 tablespoons) unsalted butter, divided
• 4 ounces coarsely grated Italian Fontina or Gruyère (about 1 cup packed)
• 1/4 teaspoon freshly ground black pepper
• 1 (2- to 2 1/2-pound) head cauliflower, cut into 1 1/2-inch pieces, including stems and core
• 3 garlic cloves, peeled and smashed
• 3/4 to 1 cup grated Parmigiano-Reggiano, divided