Clean and wash the mushrooms, cucumber and radishes and cut them into very thin slices. Sprinkle mushrooms with two tablespoons of lemon juice. Wash and drain the sprouts. Clean and wash the rocket, pluck into bite-sized pieces.
Arrange the prepared ingredients on plates. For the vinaigrette, peel and finely chop the onion. Mix vinegar, remaining lemon juice and onion. Season with salt and pepper. Whip the oil into the vinaigrette and pour over the salad.