Peel and finely chop onion, sautée in oil, mix with ground beef, season with salt and pepper and knead. Shape the beef into round patties about 2 cm thick (approximately 3/4 inch). Heat oil in a pan and fry each patty for 3-4 minutes. Rinse and pat dry the lettuce. Rinse and slice the tomatoes.
Peel the white onion and cut into rings. Slice the buns and spread the lower halves with 1 tablespoon of ketchup. Lay lettuce and tomato slices over the ketchup, then cover with a hot hamburger patty. Place the onions and 1 tablespoon of mayonnaise on each patty and place the top bun on top.