Add butter to pan and heat. Cook onions until they just begin to brown. Add in brown sugar and cook, stirring often until golden brown.
Add in balsamic vinegar and stir to combine and deglaze the pan.
Transfer onions to a bowl.
Melt remaining 2 Tbsp butter and add beans.
Cook and stir until they begin to get brown spots and reach desired tenderness. About 8 minutes or so
Salt and pepper to taste.
Add in caramelized onions and crumbled bacon.