Preheat oven to 375-degrees.
In a large skillet over medium heat, cook onion and ground beef until beef JUST browned. Add mushrooms and cook until beef is cooked through and onion is translucent. Drain excess grease.
Sprinkle flour over beef/mushroom mixture and stir until flour coats the beef.
Slowly stir in the milk, stirring constantly until you reach your desired thickness (you want the sauce to have the consistency of a thick gravy). You may or may not need all the milk.
Season with beef bouillon and salt and pepper to taste. Stir in green beans (optional).
Spray a 9x13 baking dish with cooking spray and pour beef mixture into the bottom of the dish, spreading evenly.
Evenly layer tater tots over the beef mixture.
Bake for 35-40 minutes or until tater tots become golden brown and crispy.
Remove from oven, top with cheese, and bake an additional 5 minutes.