Soak the toast. Peel and finely chop 1 onion. Squeeze the toast and knead with the minced meat, diced onions, egg and mustard. Season with salt and pepper. Shape into 4 very flat minced steaks. Fry in hot oil for 3-4 minutes on each side.
Take it out.
Drain the cucumbers well. Cut remaining onions into rings. Wash and drain lettuce and tomatoes. Cut the tomatoes into slices. Warm the rolls on the toaster or in a hot oven.
Cut the bread roll open and spread with mayonnaise. Cover lower halves with lettuce, meatballs, tomatoes, cucumber slices, onions and sauce. Place the upper halves of the roll on top.