Soak beans overnight. Bring to a boil in same water, adding more water to cover as necessary, and simmer until tender, 1-½ to 2 hours. Preheat oven to 275 degrees F. Put onion and apple in bottom of bean pot, fill half full with beans, add salt pork, and pour in remaining beans. Pour maple syrup over beans, sprinkle with mustard and salt, and cover with boiling water. Bake 5 hours.