Start by slicing onions and layering them on the bottom of your slow cooker. Cut the boneless venison roast in smaller pieces to fit into your Crock-Pot and then lay the meat on top of the onion slices.
In a bowl, mix the chipotle peppers in adobo sauce (the entire can) with the ketchup, Worcestershire sauce, apple cider vinegar, soy sauce, hot sauce, and melted butter. Whisk in the chopped garlic, brown sugar, ground mustard, celery salt, cumin, garlic powder, onion powder, and ground nutmeg.
Pour the liquid mixture over the roast. Place the lid on the slow cooker and cook on low for eight to 10 hours, until the meat is cooked all the way through and is tender. Let the meat cool a bit, then shred it with a fork to get pulled venison.