Preheat oven to 350°F; butter a shallow 9x13-inch dish.
Cook pasta just al dente; drain well and set aside.
Melt 5 tablespoons butter in a large pot over medium; whisk in flour and cook 2 minutes.
Slowly whisk in milk and cream; stir constantly about 6-8 minutes until thickened.
Remove from heat; stir in cheddar and Gruyère until completely melted.
Stir in mustard, nutmeg, salt, pepper, and garlic powder.
Fold in pasta to coat every piece.
Spread evenly in dish.
Toss panko with melted butter; sprinkle evenly over top.
Bake 30-35 minutes until top is golden brown and edges bubble; stand 5 minutes before serving.