We lived out side of Boston in a small town when I was in grammar school. I remember a neighbor bringing "Boston Baked Beans" to the house as a welcome to the neighborhood. They were so good. My mom got the recipe and handed it down to me.
Sort beans. Add cold water and bring to a boil covered. Simmer for 30 minutes.
Drain but do not discard liquid. Place onions in the bottom of a bean pot or 10 cup casserole dish.
Combine salt, vinegar (or pickle juice-my favorite), mustard, brown sugar, molasses, ketchup, and pepper and turn into pot.
Add beans and enough hot drained liquid to cover beans. Arrange salt pork on top; cover and bake at 250-degrees for 7-8 hours. Add additional liquid as needed. Remove cover the last hour of cooking so salt pork can brown.