Fry bacon and onion together until onion is soft and bacon is beginning to brown. Do not pour off bacon grease. Add mustard, vinegar and brown sugar and simmer for 20 minutes.
Place beans in a 2 1/2 quart casserole dish (works great in a slow cook crock pot as well). Add bacon mixture to beans and stir.
Bake uncovered at 350° F for one hour. Stir occasionally. If using the slow cooker, let the beans stew in the bacon mixture for at least 2 hours, actually the longer the better.
Servings: 4