Tortellini Gratinata With Mushrooms And Parsnip Béchamel

Name: Tortellini Gratinata With Mushrooms And Parsnip Béchamel
Ingredients: 12 ounces parsnips (about 2 large), peeled, cut into 1/2-inch-thick rounds; 2 1/2 cups whole milk, divided; 1 cup freshly grated Parmesan cheese, divided; Large pinch of freshly grated nutmeg; 4 1/2 tablespoons butter, divided; 1 1/4 pounds baby portobello mushrooms (portobellini), thinly sliced; 2 large garlic cloves, finely chopped; 2 teaspoons minced fresh rosemary; 3 8- to 9-ounce packages cheese-filled tortellini or tortelloni; 6 ounces imported creamy Gorgonzola dolce cheese, cut into small pieces (about 1 1/2 cups)