Beef Burgers with Tomatoes, Onion and Cheese

Peel onion, chop finely, sauté in butter and mix with ground meat. Season with salt and pepper and knead well. Chop parsley finely, mix into the meat mixture and shape into 4 round, 2 cm (approximately 3/4-inch) thick patties. Heat vegetable oil in a heavy pan and fry hamburgers 4 minutes per side. Rinse the lettuce and pat dry. Rinse tomatoes and cut into slices. Toast hamburger buns in the oven briefly and split. Spread the bottom halves with mayonnaise and drizzle the top halves with ketchup. Place a lettuce leaf, hot hamburger patty, cheese slice and a few tomato slices on the lower bun halves and set the top bun halves on the tomato slices.