Bring a large pot of well-salted water to a rolling boil.
Boil macaroni until just shy of al dente. Reserve 1/2 cup pasta water, then drain.
Stir panko with melted butter and Parmesan in a skillet; toast until lightly golden. Set aside.
Melt the butter for the sauce in a medium pot over medium heat.
Whisk in flour and cook, stirring, until blond and nutty, about 2 minutes.
Gradually whisk in warm milk and cream. Simmer, whisking, until thick enough to coat a spoon.
Season the sauce with salt, pepper, Dijon, garlic powder, and nutmeg. Adjust salt to taste.
Take off heat. Stir in Gruyere and cheddar until smooth and glossy.
Fold in drained pasta. Loosen with a splash of reserved pasta water if needed.
Serve hot, topped with toasted crumbs and a sprinkle of chives or parsley.