Preheat the oven to 325°.  Spray a 9×13 baking dish with cooking spray.

Heat a skillet over medium heat and melt the butter in the pan.  Add the chopped onions and cook until translucent.  In a large bowl, add the beans, onions, ketchup, molasses, mustard, and brown sugar.  Stir to mix well.  Scoop out into the baking dish.  Top with the slices of bacon and bake, uncovered, at 325° for 2-3 hours, until carmelized and bubbly.  When the beans come out of the oven, use a large serving spoon to gently press on the top to skim off the bacon fat.