Preheat oven (electric cooker: 220 °C/circulating air: 200 °C/gas: see manufacturer). Line baking tray with baking paper. Spread the sandwiches on the baking tray and roast in the hot oven for 8-10 minutes.
Grate cheese. Mix with mustard and sour cream. Season to taste with salt and pepper. Halve meatloaf. Wash apple, quarter, core and cut into slices.
Spread slices of bread with some mustard cream. Place meat loaf and apple slices on top. Add the rest of the mustard cream and bake. Then sprinkle with fried onions.