Step 1

Preheat oven to 350°. Cook macaroni in a 6-qt. stockpot according to package directions for al dente. Drain; return to pot.

Step 2

In a large saucepan, melt 6 tablespoons butter over medium heat. Stir in flour until smooth; whisk in warmed milk. Bring to a boil, stirring constantly; cook and stir 2-3 minutes or until thickened.

Step 3

Remove from heat; stir in Gruyere and cheddar cheeses, salt, pepper and nutmeg. Add to macaroni, tossing to coat.

Step 4

Transfer to a greased 13x9-in. baking dish. Toss bread crumbs with Parmesan cheese and melted butter; sprinkle over casserole. Bake, uncovered, 30-40 minutes or until bubbly and top is golden brown.