Preheat a grill to high. Place the mushrooms on a baking tray. Dot each mushroom with a little butter and the crushed garlic and then drizzle over the balsamic vinegar and olive oil and grill for 5 mins.
Mix the cheese and herbs together and divide between the mushrooms and grill for another 4-5 mins until the cheese has melted.
Toast the bread and serve the mushrooms on top of the toast with the watercress, spinach and rocket.
Cook's tip: for baby, purée just a little mushroom and mix with puréed potato, baby rice or baby couscous.