In a large pot boil water and follow directions to cook your choice of pasta. Or use those leftover noodles saved over in the fridge (your choice).
While waiting for the water to boil for the pasta, in a saucepan heat milk over low to low-medium heat. Add the cheeses until they melt, stirring constantly. (Be careful not to scorch or boil the elements in the saucepan).
In a separate skillet, Caramelize the finely diced yellow onion and minced garlic with non-stick cooking spray until soft and tender, showing a caramelized brown color. Pour the caramelized garlic and onions into the saucepan with the melted cheeses. Mix. To the same saucepan mix in the nutmeg, salt, & pepper.
Once pasta is al dente, drain the noodles of its pasta water. If you are using up those left over noodles, pour a bit of water in with those noodles and cover, heat on high in the microwave until warm and tender. This will make it easier to mix those cold noodles that may be a bit stiff from sitting in the fridge. Pour the melted cheese mixture in with the cooked noodles and mix until evenly coated.
In a 2 quart casserole dish spray with non-stick cooking spray. Pour the noodles and cheese mixture into the baking dish. Sprinkle over the top with bread crumbs and parmesan cheese.
Bake for 15-20 minutes until bubbly and browned on the top.
Dish and serve hot.
FYI: If you're not a fan of onions, simply omit this ingredient, it will be just as good. The onions give it a bit more flavor. If you want the flavor of onion but not the onion texture, grate the onions with a cheese grater before caramelizing, it will eliminate the chunky texture within the creamy mac n' cheese.