Grease the interior of the slow-cooker crock with butter or nonstick cooking spray.
Make the Dry Rub: In a bowl, mix together all of the dry rub ingredients.
Spread a thin layer of mustard on all sides of the ham. Spoon it on, then smear it around with your fingers. Try to cover every inch.
Then pat the dry rub ingredients over the mustard on all sides of the ham.
Place the coated ham cut-side-down into the bottom of your slow-cooker crock.
Pour Worcestershire sauce and water down along the side of the cooker so you don’t wash off any of the rub and mustard.
Cover. Cook on low 5 to 6 hours, or until an instant-read meat thermometer registers 100°F when stuck into the middle of the ham (but not against the bone).
Lift the ham onto a platter and cover it with foil. Let it stand 10 to 15 minutes so it can regather its juices.
Slice. Drizzle the slices with cooking juices from the cooker to serve.