1. In a small bowl, combine BBQ sauce and buttermilk until smooth. Add onions. For best results, soak onions overnight but this is not required.
2. Cut toppings as desired.
3. Heat oil to 375 F. Add flour in a separate bowl. Strain onions from buttermilk BBQ marinade. Toss onions in flour until well coated. Transfer to plate and lightly salt and pepper to your preference. Set aside until cheese is melting on burgers.
4.Incorporate soy sauce, worcestershire, and pepper into meat. Do not over mix, as it will toughen the meat when cooked. Form burger patties to desired size. Grill patties for 5 minutes per side on hot grill or until well done. Tip: If working on a gas grill, place a slotted metal sheet under the grates. This will stop flame-ups from drips to prevent char spots on bottom of burgers. Turn off grill, add cheese to melt.
Add onions directly to hot oil. If necessary, gently break apart with tongs. Cook until golden brown, about 1 minute for very thin onions.