Cut sesame seed buns in half horizontally. Season ground beef with salt and pepper and form four burgers. Fry burgers in hot oil, about 2 minutes per side. (Press down on burgers with a spatula at the beginning of the cooking time to flatten slightly.) Rinse tomatoes, remove cores and cut into slices.
Cut pickles into slices. Peel onion and cut into rings. Rinse and trim lettuce leaves. Layer buns with lettuce, tomatoes, pickles and onions. Top with burgers and ketchup.