In a large bowl, combine the bread crumbs, onion salt and brown sugar.
Add egg. Crumble beef over mixture and mix well. Shape into four patties.
Place in a shallow dish; pour barbecue sauce over patties. Cover and refrigerate for 2-4 hours.
In a small bowl, combine the sauce ingredients; cover and refrigerate until serving. For topping, melt butter in a small skillet. Stir in honey until blended.
Add onions; saute for 15-20 minutes or until tender and lightly browned.
Remove from the heat and keep warm.
Drain and discard barbecue sauce. Grill patties, uncovered, over medium heat or broil 4 in from the heat for 5-7 minutes on each side or until a thermometer reads 160° and juices run clear. Top each with a cheese slice; cook 1 minute longer or until cheese is melted.