Add onion and beef, brown 5-7 minutes, stirring to crumble the beef.
Add mushrooms and cook for 2-3 minutes or until they are tender. Stir in soup, milk and vegetables. Cook and stir 2-3 minutes until blended. Remove pan from the heat.
When potatoes are done, remove from oven. Top with ground beef mixture. Place pie crust over. Trim off the remaining part around your baking dish. Return to oven, bake 25-30 minutes at 400F or until crust is golden.