Boil the potatoes in salted water until tender, about 15 minutes. Drain and mash with milk and butter. Season with salt and pepper.
While the potatoes are boiling, cook the sausages in a skillet over medium heat until browned and cooked through. Remove and set aside.
In the same skillet, cook the sliced onion until soft. Sprinkle the flour over the onions and cook for another minute. Gradually add the beef broth, stirring constantly, until the gravy thickens.
Place the sausages in a baking dish, pour the onion gravy over them, and top with the mashed potatoes.
Bake in the preheated oven for 20 minutes, or until the top is golden brown.
Notes:
For a twist, add some grated cheese on top of the mashed potatoes before baking.