Make the Marinade: Place the onion, garlic, adobo sauce, olive oil, chile powder, cumin, oregano, salt, and pepper into a food processor or blender and process until smooth. Pour the marinade into a 1-cup measuring cup and add enough water to reach to 1 cup. Using a fork, poke holes all over the chicken thighs and place in a resealable plastic bag. Pour the marinade over, seal, and use your hands to move the marinade around so that all of the chicken is evenly coated. Refrigerate for at least 1 hour, or overnight. Cook the Chicken: Preheat a large skillet over medium heat. Add enough olive oil to coat the bottom of the pan. Add the chicken to the pan and cook, turning occasionally, until completely cooked through, 165 to 175 degrees F on an instant-read thermometer. (I recommend cooking in two batches and adding more olive oil in between, if needed). Remove the chicken to a large cutting board and allow to rest for at least 10 minutes. Slice or chop the chicken, as desired. Serve in a burrito, in a salad, or on its own!