1 Peel and finely chop onions. heat butter in a pan and cook onions until translucent. Mix ground beef with onions and season with salt and pepper. Shape into round patties, about 2 cm (approximately 3/4 inch) thick. Swirl in enough oil to coat a heavy pan and cook hamburgers for about 4 minutes per side. Rinse frisee and pat dry.
2 Rinse and slice tomatoes. Peel white onion and cut into rings. Rinse and dry chives. Split buns. Layer bottom bun halves with tomato slices, lettuce, chives, onion rings and hamburgers. Top burgers with cheese slices and about 1 tablespoon ketchup. Replace top halves of buns and serve.