Add sliced onions to the bottom of a 6-quart slow cooker and layer chicken breasts on top.
Unwrap the ham and discard any flavoring packet.
Check for and remove any plastic caps on the bone, if applicable.
Pour pineapple juice into the bottom of a large oval slow cooker (6-9 quarts).
In a bowl, mix brown sugar, Dijon mustard, balsamic vinegar, and honey to form a paste.
Coat the ham evenly with the paste and place it flat-side down into the slow cooker.
Cover with the lid and cook on LOW for 4-6 hours, or until the internal temperature reaches 140°F.
Optional: Flip the ham and baste it halfway through cooking, and again about an hour before it finishes.
Once cooked, transfer the ham to a platter, tent with foil, and let it rest for 15 minutes.
Skim the fat from the cooking juices and strain them into a saucepan over medium heat.
Bring the juices to a simmer.
Mix cornstarch and water to create a slurry, then whisk it into the simmering juices.
Stir until thickened; add more slurry if needed.
Serve the ham warm with the thickened glaze on the side or drizzled on top.