1 For the hamburger, soak the stale bread in lukewarm water.
2 Rinse the lettuce and shake dry.
3 Peel the onions and cut into rings. Rinse the tomatoes, remove the stalks and cut into 4 slices. Heat the oil in a pan and cook the onion rings until golden brown.
5 Mix the ground meat with the mustard, parsley, egg and the well-squeezed bread. Knead and season with salt, pepper and paprika. Form 4 hamburger patties and cook in a hot pan with brown butter on both sides. Add one slice of cheese and slide under a preheated broiler until the cheese is melted, watching carefully.
6 Toast the rolls briefly on both sides, cut horizontally in half and brush the bottom with some mustard. Place a lettuce leaf on it, add some mayonnaise and put a hamburger patty on it. Place a dollop of ketchup and cover each with a slice of tomato and some onion rings. Place the top of the bread and serve immediately.