In a saucepan, cover beans with water; cook until crisp-tender. Meanwhile, in a small jar with tight-fitting lid, combine the next six ingredients and shake well. In a bowl, combine tomatoes, onion and 1/4 cup dressing. Combine mushrooms and 2 tablespoons dressing in another bowl. Drain the beans; place in bowl. Add remaining dressing. Chill the vegetables in their separate bowls for at least 1 hour. To serve, arrange vegetables on a platter.