Begin by making your sauce: in a food processor or blender, blend red peppers, chipotle peppers, vinegar, lemon, spices, and garlic. While motor is running, add oil. Blend until smooth or no chunks remain.
For the Slow Cooker:
Add chicken to the bottom of the slow cooker. Top with chopped onion and sauce. Set your slow cooker for 4 hours on low.
Shred chicken and serve with cilantro over rice, cauliflower rice, or potatoes.
For the Instant Pot:
Select the saute function on your Instant Pot. Once hot, add avocado oil or olive oil. Saute onion for 2-3 minutes or until fragrant. Select cancel.
Add chicken, and top with sauce. Secure the lid on your Instant Pot and select the manual function. Pressure cook for 7 minutes.
Once cooking is complete, use a quick release. Shred chicken with a fork and serve hot with cilantro.