Cook beef and onion in a pan, drain fat. Season beef with garlic and oregano. Stir in soup, ketchup, pepper, mushrooms and vegetables. Mix well.
Make mashed potatoes: bring 1 Cup milk and butter to a boil, remove from heat, add potato flakes and mix well. Add 1/2 tsp garlic powder (optional).
Press meat mixture into the bottom of a pie pan, spoon potatoes over the top evenly.
Bake for 15 minutes. Broil for 2 minutes to make top of potatoes golden brown.