Wash dried navy beans under cold running water and drain. Discard any shriveled or discolored beans. Peel onions and cut into ¼-inch slices. 

Place beans in a large saucepan and add water to cover by 2 inches. Bring to a boil and boil for 2 minutes. Remove from heat and let beans soak for 1 hour. Drain. 

Return beans to the saucepan and add water to cover by 2 inches. Bring to a boil, then reduce heat and simmer, covered, until skins begin to split. Drain beans, reserving the cooking liquid. 

Preheat oven to 350°F. 

In a large bowl, combine brown sugar, salt, dry mustard, and molasses. Stir in 4 cups of the reserved bean cooking liquid, adding additional water if needed to reach 4 cups. 

Place beans, salt pork, and onions in a baking dish. Pour sauce over beans and cover dish. Bake for 3½ hours, adding water as needed during baking to keep beans soupy. 

Ladle hot beans and sauce into a hot jar, leaving 1-inch headspace. Remove air bubbles. Wipe jar rim clean. Center lid and apply band, adjusting to fingertip-tight; repeat until each jar is filled.  

Place jars in a pressure canner with 2 inches of simmering water (180°F). 

Process pint jars for 80 minutes or quart jars for 95 minutes at 10 pounds pressure, adjusting for altitude. Turn off heat and allow canner to cool to zero pressure. Let jars stand 5 minutes before removing. Cool 12 hours. Check seals, label, and store. Enjoy!