Place onion in a medium microwave-safe bowl; cover loosely with heavy-duty plastic wrap. Microwave on HIGH 2 minutes. Stir garlic and next 5 ingredients into onion.
Roughly tear cheese slices; place in a 4-quart slow cooker. Pour milk over cheese; add onion mixture. Cover and cook on LOW 2 hours. Stir gently to blend ingredients.
Make Ahead: Spoon dip into serving-size freezer containers, and freeze up to 1 month. Thaw overnight in refrigerator. Reheat in microwave on MEDIUM (50%) power.