Preheat oven to 350°F and butter a 9x13-inch shallow baking dish.
Cook pasta until just al dente, drain well, and set aside.
In a large pot, melt 5 tablespoons butter over medium heat, then whisk in flour and cook for 2 minutes.
Slowly whisk in milk and cream, stirring constantly until sauce thickens, about 6-8 minutes.
Remove from heat and stir in white cheddar and Gruyère until completely melted.
Add Dijon mustard, nutmeg, salt, pepper, and garlic powder, stirring to combine.
Fold in cooked pasta until every piece is coated in sauce.
Pour into prepared baking dish and spread into an even layer.
Toss panko with melted butter and sprinkle evenly over the top.
Bake for 30-35 minutes until top is golden brown and edges are bubbling, then let stand 5 minutes before serving.