Drain beans and reserve 1 cup of the water beans were cooked in. Set aside.
Fry bacon until crisp and drain on paper towel.
Sauté onions in bacon fat until golden brown.
In small bowl, mix cornstarch, salt, mustard, brown sugar and vinegar.
Add cup of reserved water from beans and beat until smooth, then add mixture to bacon fat and onions in pan and boil until thickened slightly.
Add beans and stir well.
Crush bacon and sprinkle on top. Serves 8.