For this burger, which has become what we call a "pub favorite" in America, we again combine ground chuck with ground sirloin. If you prefer, you can use one or the other, but remember that sirloin alone will be a little drier than the combination. However, the bacon, although cooked, and the mushrooms add a little moisture. The chuck alone will produce a juicy and plump burger, but will lack a little of the deep flavor of the sirloin.
Cook the bacon over medium heat in a skillet until cooked by not crispy. Drain on paper towels and when cool enough to handle, tear or chop into small pieces.
Combine the beef, bacon, onion, mushrooms, salt, and pepper in a large bowl. Using your hands, mix well. Form into 6 patties. Refrigerate until ready to grill.
Prepare a charcoal or gas grill. Lightly spray the grill rack with vegetable oil cooking spray. The coals should be hot.
Grill the burgers for about 5 minutes. Turn and grill for 4 or 5 minutes longer for medium-well burgers.