Layer diced onions in bottom of slow cooker.
Add chicken thighs on top of onions and spread out.
Pour entire can of tomatoes and green chilies over chicken.
Sprinkle all spices and garlic over everything without stirring.
Cook on high for 3-4 hours or low for 6-7 hours until chicken reaches 165F and shreds easily.
Remove chicken to cutting board and shred with two forks.
Add 1/4 to 1/2 cup cooking liquid to shredded chicken.
Return chicken to slow cooker with additional liquid to keep warm until serving.