1Cook the pasta according to package directions. I recommend small shell pasta because cheese gets inside the shells and tastes good. When the pasta is just tender, drain. Don't overcook.
2Melt butter in the pan. Add the onion and saute until tender. Reserve a couple teaspoons of the butter from the pan for the crumb topping. Add the milk to the butter, then add cornstarch, nutmeg, salt and pepper and whisk until smooth. Add the cheese soup. Whisk again. Remove from heat.
3Pour the hot cooked and drained pasta shells into a greased glass baking dish and add the grated sharp cheddar cheese. Stir.
4Then pour the hot pan mixture over the shells and gently mix until combined.
5Make the topping:
Put the 4 tablespoons bread crumbs in a small bowl and add the reserved 2 teaspoons of butter and mix well. Sprinkle over the macaroni and cheese and bake in a 375 degree oven for 30 minutes or until the topping is brown.