Combine grilled onions, bacon, brown sugar, syrup, and apple cider vinegar and cook down until a jam like consistency is formed. Set aside some bacon fat for sauce.
Smash balls of meat and season. Flip and add cheese. Add butter to griddle, put down your burger bun face down followed by sauce, cheese, burgers, bacon jam, more sauce, more cheese, and the top bun. Smash to grill then flip to grill the other side.
