Rinse the dried navy beans under cold water and soak them overnight in a large bowl of water.

Drain the soaked beans and transfer them to a large pot. Cover the beans with fresh water and bring to a boil. Lower the heat and let them simmer for about 1 hour, or until they are tender.

In a separate bowl, mix together the brown sugar, molasses, diced onion, mustard, salt, and black pepper to create the sauce.

Once the beans are cooked and tender, drain them and combine them with the sauce mixture in an oven-safe pot or baking dish. Stir until well coated.

Cover the bean mixture with a lid or aluminum foil and place it in a preheated oven at 325°F (165°C). Bake for about 2 to 3 hours, checking occasionally and adding water as necessary.

Remove the cover for the last 30 minutes of baking to allow the beans to brown slightly.

Once done, take out the baked beans and let them cool slightly before serving.