Combine molasses, vinegar and mustard and mix well.
Empty beans into a flat 2-quart casserole and stir in molasses mixture.
Arrange onion slices and frankfurters on top of beans.
Bake in a 375° F.oven 35 minutes.
This casserole can be prepared in advance, refrigerated, and baked at serving time.
Add 15 minutes to baking time.