Rinse beans under running water, place in a large kettle with water. Bring to boiling, cover kettle, lower heat and cook 15 minutes. Let stand 1 hour.
Trim all excess fat from beef. Brown meat on all sides. Place meat in the bottom of the slow cooker; add beans and liquid, onion, mollasses, salt, ginger, mustard, pepper and bay leaf. Add more water, if needed to cover meat and beans; cover with lid. 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.