Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). Line a baking tray with baking paper. Grate the cheese finely and sprinkle 8 circles (approx. 7 cm Ø each) on the tray, leaving some space between the circles. Bake in a hot oven for about 6 minutes. Remove the baking tray and let it cool down on a cake rack.
Clean, wash and drain the salad and cut into fine strips. Peel and chop the onion. Clean the mushrooms, possibly wash and halve them.
Heat the oil in a large frying pan. Fry the mushrooms in it while turning. Fry the onion briefly. Season with salt and pepper and sprinkle with 1 tsp. sugar. Deglaze with vinegar and boil up briefly. Remove from the heat and add about half of the salad to the mushrooms.
Wash parsley and shake dry, chop finely. Distribute the rest of the salad in four bowls or on plates. Place 1 parmesan cracker on each plate. Spread the mushroom and salad mix on top. Place 1 parmesan cracker on each and sprinkle with parsley.