Season beef with salt and pepper. Form into 8 thin patties about 5-inches in diameter. Place 2 slices cheese and 2 tablespoons bacon in the center of 4 patties. Top with remaining patties. Press edges together, pinching to completely seal.
Prepare grill to medium heat. Place burgers on grill. Grill, turning once, about 7-14 minutes or until internal temperature reaches 160°F on an instant read thermometer. Brush with barbecue sauce the last couple of minutes.
Meanwhile, in medium skillet, heat oil over medium-high heat. Add onions and mushrooms; salt and pepper to taste. Sauté until onions are golden brown and mushrooms are softened (about 5-7 minutes).
Place burgers on rolls; top with mushrooms and onions. Serve with additional barbecue sauce, if desired.
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