Preheat the oven to 325 degrees F.
Heat the oil in pan over medium heat, then add the bacon and cook to render the fat, but remove before crisp, 2 to 3 minutes. Place the rendered bacon on paper towels or a kitchen towel to drain.
Add the garlic and onions to the same pan and stir. Cook to soften, 4 to 5 minutes, then add the tomato sauce, stock, sugar, Worcestershire, mustard and vinegar. Season with black pepper and bring to a simmer. Add the beans and stir, then transfer to a casserole dish and arrange the bacon on top. Bake the beans for 2 hours.
Preheat the griddle. Halve the knockwurst and butterfly the pieces by cutting across but not all the way through. You will end up with 12 split, halved dogs. Grill to crisp on both sides. Arrange the knockwurst in the casserole dish.
Cool, cover and store for a make-ahead meal. Reheat in a moderate oven. Garnish with scallions and sever with extra mustard.
Soak the beans in water overnight. Rinse and pick through beans for stones or imperfect beans. Place in a pot and cover with fresh water, then add the bay leaf and bring to a boil. Season with salt, and cook beans at rapid simmer until tender, 35 to 40 minutes. Drain and reserve.
You can use canned beans if you don't want to cook them from dried. Simply skip the sub-recipe step, partially drain the beans and stir into the barbecue sauce before transferring the beans to the casserole dish.