Peel the onion, finely chop, sauté in butter, mix with ground beef, season with salt and pepper, knead and form into 4 patties star-shaped (by using a greased start-shaped baking dish). Heat a little oil in a heavy pan and cook the patties 4-5 minutes on each side.
Cut the buns also into star shapes or cut to the size of the patties. Place the hot patties on the bottom buns, top each with 1 tablespoon ketchup and the top bun. Serve with potato chips, if desired.