Preheat an oven to 400 degrees F (200 degrees C).

Melt butter in a large skillet over low heat. Add onion and cook until soft and translucent, 10 to 15 minutes. Transfer to a 2-quart casserole dish and set aside. Cook bacon in the same skillet over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon on a paper towel-lined plate. Cut bacon into bite-sized pieces and set aside.

Combine baked beans, ketchup, Worcestershire sauce, and mustard in the casserole dish. Stir in 2/3 cooked bacon and 1/4 brown sugar until mixture is combined. Top bean mixture with remaining bacon and sprinkle with remaining brown sugar.

Bake in the preheated oven until hot and bubbly, about 45 minutes.