Step 1

Place the navy beans into a large container and cover with several inches of cool water; let stand 8 hours to overnight. Drain and transfer beans to a pot; add enough water to cover beans.

Step 2

Bring water to a boil, reduce heat, cover pot, and simmer until beans are slightly tender, about 1 hour.

Step 3

Mix 4 cups water, molasses, brown sugar, salt, mustard, and pepper together in a bowl until smooth.

Step 4

Mix beans, salt pork, and onion together in a slow cooker. Add molasses mixture and more water to cover beans if needed; stir well.

Step 5

Cook on Low for 8 to 10 hours.