Add the macaroni to boiling water and cook until al dente. Heat the oven to 350 degrees.
Place the beef and onion in a skillet over medium high heat, and saute until the onion is softened and the meat is cooked through. Drain off the excess grease.
Stir in the garlic powder and Italian seasoning as well as salt and pepper to taste.
Mix in the soup, milk and mushrooms. Add the macaroni to the skillet and stir to combine.
Layer the mixture in a 9x13 baking dish. Sprinkle with the cheese and bake for 20 minutes.