Finely dice onions and sauté in oil, add mushrooms and fry everything for 10 minutes.
Clean the lollo rosso salad and distribute it in portions on plates. Pluck the basil from the stems (put some leaves back), roughly chop.
Season the fried mushrooms in the pan with salt, pepper, vinegar and mustard. Fold in the basil. Arrange the mushrooms on the salad. Garnish with the basil leaves.