1 large onion, quartered and sliced
2 tablespoons butter or margarine
2 cups diced leftover roast beef
2 cups brown or beef gravy
1/2 cup sliced or diced cooked carrots
1 cup frozen peas, cooked
Salt and pepper, to taste
1 egg yolk
2 cups leftover mashed potatoes
Melt butter in a heavy skillet over medium heat; add onions. Sauté onions until tender; add diced beef, gravy, carrots and peas. Heat through; season with salt and pepper, to taste.
Transfer to baking dish. Beat the egg yolk into potatoes and spoon potatoes over the meat and vegetables (press potatoes into the pan with a pastry tube, if desired). Bake potpie at 400 degrees for about 30 minutes or until mashed potatoes are browned and gravy is bubbling.