By baking this hash in the oven, you eliminate the need to turn it with a spatula and constantly watch it as it cooks. Serve topped with a poached egg or with a green salad and French bread.
Preheat oven to 400° F. Melt the butter in a large cast-iron skillet and stir in the onion. Sauté over medium heat for 5 minutes. Add the potatoes and beef, and top with the gravy. With a fork, stir the mixture together gently and mash it lightly to smooth the top. Season liberally with salt and pepper. Bake for 20 minutes.
Note: If you need to cheat and make more gravy, melt 2 tablespoons of butter in a medium saucepan over low heat. Sprinkle in 2 tablespoons of flour and stir with a whisk to blend. Cook until bubbling, about 3 or 4 minutes, making sure the mixture does not burn. Add 1-½ cups of chicken stock all at once, and whisk until smooth. Increase the heat to medium and cook for 10 minutes, stirring occasionally, until the gravy is thickened and smooth. Season with salt and pepper.