Cut the hats into thin plates. I used onions, it is better to use red onions. Onions cut into thin half-rings. For the sauce, mix the vinegar, oil, salt and pepper. In a bowl lay down their sliced hats and onion and pour over sauce. Mix well, cover the bowl with film and put overnight in the fridge.
Chop the walnuts. Put arugula on a plate, put mushrooms and onions on it, sprinkle with chopped nuts.