Combine all the sauce ingredients in a high speed blender or food processor and blend until smooth.
Lay the chicken breasts on the bottom of the slow cooker and top with onion slices.
Pour the tinga sauce over the top. Slow cook on HIGH for 4 hours. See notes below for reducing cook time.
When the chicken has reached a shreddable consistency, remove the the chicken breasts and shred on a cutting board. If the tinga sauce is liquidy, reduce it by half either using sear/sauté on the Ninja Foodi/Instant Pot or a saucepan on the stove.
Toss the chicken with the sauce and serve. *If you have the Ninja Foodi, use the crisping lid on AC 400F for about 5-10 minutes for a slight char on the chicken. It's delicious that way.
Notes
To make Chicken Tinga in 2 hours instead of 4, cut the chicken breasts in half through the middle and then in half lengthwise. The smaller and thinner pieces will cook to a shreddable consistency in 2 hours on HIGH.