Place lima beans in a large bowl and cover with several inches of water; soak 8 hours to overnight.
Bring water, ham bone, and onion to a boil in a pot. Reduce heat, cover, and simmer until onion is tender, about 1 hour.
Drain and rinse lima beans and add to the pot with the ham bone. Add brown sugar, Worcestershire sauce, hot sauce, dry mustard, and salt. Simmer, covered, about 2 hours. Uncover and continue simmering soup until beans are tender, about 30 minutes more.
Remove ham bone from soup; cut the meat into bite-sized pieces. Discard bone and return meat to the soup.
