Heat oil in skillet and brown the ground beef with the chopped onion.
Preheat oven to 425 degrees. Grease or spray oil in an oven proof casserole dish. Add the cooked macaroni and meat/onion mixture.
Whisk together milk and eggs; add the nutmeg and about half of the cheese. Pour this mixture over the macaroni and meat in the casserole. Top with the remaining cheese and bake in the preheated oven for approximately 40 minutes, or until the cheese is golden brown and the casserole is heated throughout.