1. Add the olive oil to a medium frying pan on the stove over medium-high heat (or use your slow cooker insert, if possible). After a few minutes, add the diced onion and sauté for 2-3 minutes, or until the onions begin to turn translucent. Add the garlic and fry for an additional minute, then add the chicken thighs. Brown the chicken thighs on one side for 4-5 minutes before turning and browning on the other side for the same amount of time. Scrape everything into the slow cooker, or if using the insert, place the insert back into the base. Add the diced tomatoes.
2. In a food processor or high powered blender, combine the remaining ingredients and blend on high until pureed and smooth. Taste and adjust seasonings if necessary. Pour the sauce over the chicken mixture in the slow cooker, and give everything a stir. Turn the slow cooker to low and cook for 6-7 hours (or 3-4 on high).
3. Remove the chicken from the slow cooker, and using two forks, shred it into long strands, returning it to the slow cooker. Stir everything together and cook on low for an additional 10-15 minutes, then serve over rice, couscous, or quinoa with a bit of lime and feta; or, serve as chicken tinga tacos with warmed flour tortillas, feta and cilantro.

Leftover chicken tinga can be kept in the fridge, in an airtight container, for 3-4 days, or frozen for up to 2 months.