In a large stockpot over medium heat, cook the bacon until lightly browned and most of the fat has rendered, about 5-7 minutes. Add the diced onion and sauté until the onions are soft, another 5-7 minutes. Add the baked beans, kidney beans, brown sugar, ketchup, and mustard; stir until combined. Reduce the temperature to medium-low to maintain a low and gentle simmer. Cover the beans and cook for 1 hour, stirring occasionally to keep the beans from sticking to the bottom. Uncover the beans and continue to cook for another 30-45 minutes, stirring occasionally, until the liquid has reduced and the beans have thickened. Stir in the pepper and taste for seasoning, adding more pepper if needed. Enjoy!