Once onions are clear (or caramelized if you prefer), open the cans of baked beans. Don't shake the cans! Strain off the extra liquid at the top of each can. Dump just the baked beans into the pan over the onions.
Next, add the amount of brown sugar you desire, your choice of BBQ sauce, and the dry mustard. Stir that well. Place the lid over the beans mixture and cook on low for 2 hours.
You will want to stir your beans every half hour to see how thick they have become and make sure none have burned to the bottom of your pan. If some have burned to the bottom, leave them there if you don't want that taste in your beans. Stir just above them. If you like crispies in your food, stir them up into your beans.
The beans are ready to serve when they are as thick as you want them. Enjoy!
Notes
If you want to turn this into comfort food, brown a pound of ground beef, stir that into your beans, and serve hot over fresh buttermilk biscuits. It's delicious!