Rinse and pick through beans thoroughly. (Remove any bad beans or small rocks.)

Combine beans, onion, dark brown sugar, molasses, apple cider vinegar, mustard, water, and black pepper in a 6 quart slow cooker. (Add bacon if using.)

Stir just until the ingredients are combined. Cover and cook on low until the beans are tender, about 12 hours.

Stir in salt to taste and additional pepper if desired. Start with half a teaspoon and adjust as needed.

Turn slow cooker to warm until ready to serve.