In a skillet over medium-high heat, cook the ground beef and onions with 1/4 cup bouillon until the meat is browned and fully cooked. Drain and season with salt and pepper to taste.
Transfer the meat and onions to an 8x11-inch baking pan and cover with a layer of peas and carrots.
Prepare 2 cups of medium-thick gravy using bouillon to flavor. Pour the hot gravy over the meat and vegetables.
Combine the leftover mashed potatoes and the milk and mix well. Season with salt and pepper to taste. Spread the potatoes over the meat mixture and dot the top with butter.
Bake in the preheated oven for 1 hour.
Recipe Tips
For traditional sheperd's pie, use ground lamb instead of beef.