Preheat oven to 375°F (190°C). Grease a 9×13-inch baking dish.
In a bowl, gently mix all hamburger steak ingredients until just combined. Form into oval patties.
Arrange sliced potatoes evenly in the baking dish and lightly season with salt and pepper.
Place hamburger steaks on top of the potatoes and scatter onions around them.
In a bowl, mix soup (or gravy) with milk/broth. Pour evenly over the dish.
Cover tightly with foil and bake for 50–55 minutes, until potatoes are tender and beef is cooked through.
Uncover and bake 10–15 minutes more to slightly thicken the sauce.
Rest 10 minutes before serving.