Sort the lamb`s lettuce, wash and spin dry. Cut the onions into rings. Quarter, core and slice apples. Clean the mushrooms (do not wash !!!) and cut into slices. Drizzle the mushroom and apple slices with the juice of 1 lemon. Mix all ingredients together.
For the marinade, mix vinegar with mustard, pressed garlic clove, salt, pepper and sugar. Stir in 4 tablespoons of oil and add 2 tablespoons of chopped walnuts. Just before serving, pour the marinade over the salad and mix in well.