Place onion in a slow cooker; add beef, cutting to fit if necessary.
Sprinkle with salt, pepper, and garlic.
In a medium bowl, combine cranberry sauce and chipotle chili peppers, and pour overall.
Cover and cook on LOW for 6 to 8 hours or on HIGH for 3 to 4 hours.
Garnish with jalapeno peppers, if desired, and enjoy!