Combine ground beef, onion, cheese and TABASCO® Buffalo Style Sauce in a mixing bowl and mix just until well blended; shape into four patties. Brush mushrooms with oil.
Grill burgers and mushrooms over medium-high about 4 minutes per side or till burgers are of desired doneness and mushroom caps are soft. Just before removing burgers from grill, top each with a slice of cheese, then place burgers on bottom half of buns; top each with a tomato slice, a grilled mushroom, and a bun top. Serve with additional TABASCO® Buffalo Style Sauce, if desired.
Makes 4 servings.