In the bottom of a 5 to 6 quart slow cooker, sprinkle half of onions. Crumble beef over onions; top with remaining onions and garlic.
In a small bowl, stir together chili powder, cumin, paprika, salt, garlic powder, and allspice; sprinkle half of spice mixture over ingredients in slow cooker.
Add sweet potato, chile peppers (if using), beans, tomatoes, kale, broth, and tomato paste. Sprinkle with remaining spice mixture.
Cover and cook on high for 6 to 8 hours, stirring once to combine ingredients.
Before serving, stir in apple cider vinegar. Taste, and add additional salt, if desired.
Garnish with sour cream and green onion, if desired.