Combine Hellmann’s® or Best Foods® Canola Cholesterol Free Mayonnaise, vinegar and honey in small bowl; reserve 2 tablespoons. Brush mushrooms and onion with remaining mayonnaise mixture. Grill, turning once, until vegetables are tender, about 10 minutes.
Spread reserved mayonnaise mixture on 4 muffin halves. Top with onion slices, mushroom, lettuce and tomato, then remaining muffins. Serve warm.