Season the chicken with salt and pepper. Place it in the Slow Cooker.
Heat the olive oil in a small skillet over medium-high heat, add the sliced onions, and saute’ until translucent, about 8-10 minutes.
Transfer the onions to the slow cooker.
Add the tomatoes, garlic, cumin, oregano, and chilis in adobo sauce, plus extra adobo sauce, to a blender or food processor. Puree’ until blended into a smooth sauce.
Pour the sauce over the chicken and onions. Cook on high for 2-3 hours or low for 4-6 hours.
Once the chicken is cooked through, remove the thighs to a cutting board and shred with two forks.
Return the meat back to the slow cooker and stir it into the sauce. Let it simmer for an additional 20-30 minutes on the low or warm setting.
Serve over tortillas with red onion, cilantro, and cheese.
