Cook beans. Place cooked beans into a crock pot or large casserole dish. Gently stir salt pork or bacon into the beans. Combine mustard, brown sugar, molasses and pepper; pour over beans in crock and stir gently to mix.
Add just enough water to cover, about 1 cup. Gently push quartered onion into the beans. (Do not add vinegar or salt at this time, or beans will not soften.)
Cover, and bake in a 300°F (150°C) oven for 4 to 6 hours, until done to desired consistency, adding a little boiling water if beans appear to dry out. Remove cover, stir in vinegar and salt, and cook for 1 more hour, without adding water.
Makes 6 servings.