1. Place Portabellas and vinaigrette in a large zip top bag. Zip and lightly rub the vinaigrette into the mushrooms. Let marinate for at least 30 min.
2. Remove mushrooms from bag, drain and season both sides with salt and pepper.
3. Heat a grill or grill pan over medium heat and spray with non-stick cooking spray. Place the mushrooms on the grill gill side down and cook for 4 minutes. Turn and continue to grill until mushroom is almost cooked through, about 4 more minutes.
4. Remove from pan and drain mushrooms on paper towel, gill side down. Cut each mushroom into quarters and place on buns. Top with onion and tomato and serve.
Recipe and photos courtesy of the Mushroom Council and mushroominfo.com