Take a shortcut and start with a can of Boston-style baked beans and add smoked bacon, onion, ketchup, brown sugar, and mustard for an easy bean side dish.
In a 5- to 6-quart pan over medium-high heat, stir bacon often until browned and crisp, 6 to 8 minutes. With a slotted spoon, transfer to paper towels to drain. Discard all but 1 1/2 tablespoons fat in pan.
Add onions to pan and stir often until they begin to brown and stick to bottom of pan, 7 to 9 minutes. Add beans and their liquid, catsup, brown sugar, dry mustard, and cooked bacon; mix well. Pour into a shallow 3-quart casserole.
Bake, uncovered, in a 350° regular or convection oven until beans are bubbling in the center, 30 to 40 minutes. Serve hot.
If assembling through step 2 up to 1 day ahead, cover and chill; bake chilled beans about 50 minutes.