Preheat oven to 350°F.
Cook hamburger with onion and butter. DO NOT DRAIN (unless you used a more fatty beef)! Add cream of mushroom and milk.
Cook over medium/medium-high heat until smooth. Remove beef from heat.
Starting with the prepared macaroni and cheese, layer beef mixture and macaroni. Make 2 layers of each.
Be sure that macaroni is covered well by the beef mixture and its' "juice" or it will dry out when cooking. Cook for about 25 minutes or until bubbly.
Submitted by: nfe