Soak rolls in water. Peel and finely chop the onion. Knead minced meat, squeezed out roll, egg, onion, salt, pepper, paprika and 1 teaspoon of mustard with the dough hooks of the hand mixer.
Form four meatballs from the minced dough. Heat the oil in a frying pan and fry the meatballs for about 12 minutes over medium heat, turning them over. Take the meatballs out of the pan and cut them lengthwise.
Clean, wash and slice the tomatoes. Wash lettuce, dab dry and pluck a little smaller. Coarsely grate cheese. Chop cucumbers into thin slices. Mix mayonnaise and remaining mustard.
Cut the baguette into four pieces of the same size and then slice. Spread the lower half of the baguette with mustard mayonnaise. Cover with lettuce, cucumber and tomato, sprinkle with cheese and fried onions and place the baguette lid on top.
Serve on a plate.