Grill burgers on barbecue preheated to medium-high for 5 to 6 min. per side or until cooked through and internal temperature reaches 160°F (71°C).
Meanwhile, cook mushrooms in olive oil in a large skillet over high heat. Add garlic, steak spice, salt and pepper and cook 8 to 10 min. or until golden and tender. Add balsamic vinegar and cook 1 min.
Assemble burger patties in buns with mayonnaise, ketchup, cheese and mushrooms.