Pick over beans and rinse. Place in a large bowl; add water to cover. Let stand overnight; drain. Combine beans and onions in a large saucepan; add water to cover; heat to boiling; cover. Simmer 45 minutes, or until skins of beans burst when you blow on several in a spoon.
Drain liquid into a small bowl. Measure 1 cup of the bean liquid and combine with molasses, mustard, brown sugar, and salt in a bowl. Layer half of the salt pork and all of the beans into an 8 cup bean pot. Pour molasses mixture overtop; add just enough more saved liquid to cover beans. Top with remaining salt pork, pressing down into liquid; cover.
Bake at 300F 4 hours; uncover. Bake 1 hour longer, or until beans are deep brown, tender, and as dry as you like them. (After 2 hours baking, check beans, and if they seem dry, add more saved bean liquid to keep them moist.)