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