Preheat the oven to 350°F. Spray (or butter) a 9x13 inch casserole dish with non-stick cooking spray. Set aside.
Cook the noodles until just al dente, according to the package directions. Drain then add 2 tablespoons of butter and stir into the hot noodles.
Melt 4 tablespoons of butter, then stir in the flour, garlic powder, onion powder, salt, pepper, and nutmeg and cook for one minute.
Whisk in the milk and heavy cream and cook until the sauce has thickened.
Stir in the shredded Gruyere and mozzarella cheese until melted and smooth. Pour the sauce over the noodles, then spread them into the prepared dish.
Using a food processor or plastic zip top bag with a rolling pin, crush the French fried onions until they are very small crumbs. Evenly spread over the top of mac and cheese.
Bake for 25 minutes, until hot and bubbly. Let stand for 5 minutes, then serve!