Make the sauce and marinade by mixing everything together in a large bowl, then put a few spoonfuls aside. Slice halfway into the thickest part of each breast and open it up like a book. Flatten down slightly with your hand, then toss in the bowlful of marinade to coat. Chill for as little, or as long, as you have time to.
Barbecue the chicken for about 10 mins until completely cooked through, turning so it doesn’t burn but is nicely charred and sticky. Cook the bacon at the same time until crisp, if using, and toast the buns. Assemble the burgers with lettuce, slices of tomato, cheese, onion and the reserved sauce on the side for dolloping on top.