Arrange the onions, carrots, and mushrooms in the bottom of a slow cooker. Rub the venison rump roast with garlic, black pepper, and salt, then place on top of the vegetables. Mix the cream of mushroom soup, beef broth, water, and onion gravy mix in a bowl until gravy mix is dissolved; pour soup mixture over the roast.
Cook on High setting until roast is very tender, 8 to 10 hours. Slice roast before serving with gravy and vegetables.