Preheat the grill to medium-high heat.

Heat the olive oil in a large skillet over medium-high heat. Once hot, add the shallot and mushrooms; saute until softened and browned, 4 to 5 minutes. Stir in the BBQ sauce and stir, while cooking, for 1 to 2 minutes more.

In a large mixing bowl, break up the meat with your hands. Add the chopped onion, garlic, Worcestershire sauce, and salt and pepper. Mix until thoroughly incorporated. Divide the meat into 6 equal balls. Flatten them into patties and use your thumb to create an indent in the center of each burger. Grill the burgers, uncovered, for 4 minutes on the first side. Flip and continue cooking for another 4 minutes for medium. Remove the burger from the grill, place in buns, and immediately top with the Swiss cheese and the hot BBQ mushrooms.

Cook's Note: The indent in the burger will prevent the burgers from puffing up while you grill them.