Spray 4- to 5-quart slow cooker with cooking spray. Place onions in slow cooker. In small bowl, mix dried minced garlic, cumin, salt, pepper and 1 tablespoon of the margarita mix to form paste. Pat mixture on top and sides of roast (if roast comes tied or in netting, remove before rubbing with spice mixture). Place roast on onions. Pour remaining margarita mix around roast.