Mashed Potato and Cauliflower Gratin

Name: 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