Heat oil and butter in a skillet over medium heat; cook chopped onion until tender and browned. Add meat and seasonings; brown, stirring to break up. Drain off excess fat. Stir in beans and soup; transfer to a greased 9-inch square baking dish. Mash the potatoes with the 1/2 cup milk and the beaten egg, along with 1/2 teaspoon salt and the pepper. Spoon potatoes in mounds over the beef mixture. Bake at 350° for 30 minutes.
Hamburger pie recipe serves 4 to 6.