1 Peel and finely chop onions and fry in oil. Mix with ground beef, season with salt and pepper and knead well. Shape into 2 cm (approximately 3/4 inch) thick patties.
2 Heat oil in a heavy skillet. Add hamburgers and fry 4 minutes per side. Rinse lettuce and pat dry. Rinse and slice tomatoes.
3 Peel onion and cut into rings. Slice buns. Layer bottom halves of buns with lettuce, burgers, and tomato and onion slices. Top each with a tablespoon ketchup and remaining bun halves.