Mix the vinegar, oil, pressed garlic, salt and pepper into a vinaigrette.
Sort the salad, wash it and spin dry. Peel and finely dice the onion.
Arrange the salad on a platter and sprinkle with the onion cubes.
Clean and quarter the mushrooms. Heat the remaining oil in a pan and fry the mushrooms in it. Season with salt and pepper and spread on the salad while still warm. Drizzle over the vinaigrette and serve immediately.