Peel and finely dice 1 onion. Knead minced meat and onion. Season with salt and pepper. Form 6 flat meatballs from the minced meat and fry in hot oil on both sides for about 5 minutes. Cut the cucumbers into slices. Peel remaining onion and cut into rings. Cover one minced steak each with some cucumber slices and onion rings.
Spread ketchup and mustard on top. Cover each with a slice of cheese. Place the minced steaks and rolls next to each other on a baking tray and bake in a preheated oven (electric range: 200 °C/ gas: level 3) for about 3 minutes. Cut the bread roll in half. Cover the lower halves with salad and add the minced steaks au gratin. Cover with the upper half of the roll