This is a great do ahead recipe that is sure to thrill all of the mushroom fans in your life. Not only can you make these far in advance, they actually taste better if they marinate for a few days.
Place the mushrooms in a flat serving dish. Place the rest of the ingredients, except the red and green pepper flowers into a small heavy saucepan. Bring the ingredients to a boil and cook until the onions soften. Pour the marinade over the mushrooms and arrange the onion slices around the mushrooms attractively. Let the mushrooms marinate at room temperature for 4 hours, stirring frequently. Refrigerate for at least 24 hours. Garnish with red and green pepper flowers.