Rinse beans under running water, place in a large kettle with water; bring to boiling; cover kettle; lower heat; cook 15 minutes; let stand 1 hour.
Trim all excess fat from beef; brown meat on all sides in remaining fat in a large skillet.
Place meat in the bottom of the slow cooker; add beans and liquid, onion, molasses, salt, ginger, mustard, pepper and bay leaf. Add more water, if needed, to cover meat and beans; cover.
Cook on high for 2 hours, stir beans, adding more liquid if needed to keep beans and meat covered.
Turn heat control to low and cook for 8 hours, or until beans are very tender and liquid is absorbed. Taste, and season with a spoonful of hot prepared mustard, if you wish.
Remove meat to a carving board and cut into slices; spoon beans around beef on platter.
