Melt the butter in a skillet over low heat. Cook and stir until the onion has softened and turned translucent, 10 to 15 minutes. Meanwhile, place the bacon in a large, deep skillet, and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain the bacon slices on a paper towel-lined plate. Cut bacon into bite-sized pieces and set aside.
Combine the baked beans, Worcestershire sauce, ketchup, mustard, and onions in a 2-quart casserole dish. Stir in 2/3 of the cooked bacon and 1/4 of the brown sugar until evenly mixed. Cover the bean mixture with the remaining bacon, and sprinkle with the remaining brown sugar.
Bake in the preheated oven until hot and bubbly, about 45 minutes.