Soak rolls in water. Peel and finely dice onions. Squeeze out the buns. Put minced, squeezed out roll, diced onion, egg and mustard in a bowl. Season with 1 teaspoon of salt and pepper.
Knead carefully, season again. Form 10 meatballs from it. Heat the oil in a large pan. Fry the meatballs in it at medium heat, turning occasionally for 8-10 minutes.