Heat 2 tablespoons of the oil in a large frying pan, add the onions and garlic and cook until soft and starting to brown. Add the mushrooms and tomatoes and cook, stirring gently, for 4–5 minutes. Remove from the heat.
Transfer the mushroom mixture to a serving dish and garnish with the olives.
Whisk the remaining oil with the vinegar in a small bowl, season to taste with salt and pepper and drizzle over the salad. Garnish with the chopped parsley, cover and leave to stand at room temperature for 30 minutes to allow the flavours to mingle before serving.