Combine ground beef with onion in medium bowl; shape into 8 patties.
Heat oil in large nonstick skillet over medium-high heat and cook mushrooms, stirring occasionally, until mushrooms are golden, about 7 minutes. Remove from heat and stir in 1/4 cup Wish-Bone® Cheddar Bacon Ranch Dressing; set aside.
Grill or broil burgers, turning once, until done, about 6 minutes. Top buns with tomatoes, burgers, mushroom mixture and remaining 1/2 cup Dressing.