Prepare the marinade: In a food processor or blender, combine the coarsely chopped ½ medium red onion, 3 cloves coarsely chopped garlic, 2 tablespoons adobo sauce, 1 tablespoon of the 2 tablespoons olive oil (reserve the other 1 tablespoon for cooking), 2 tablespoons ancho chile powder, 1 teaspoon ground cumin, 1 teaspoon dried oregano, 1 tablespoon salt, and ½ teaspoon black pepper. Process until smooth.

Dilute the marinade: Pour the blended marinade into a 1-cup measuring cup and add water as needed to bring the total volume to 1 cup. Stir to combine.

Marinate the chicken: Using a fork, poke holes all over 3 to 4 pounds boneless skinless chicken thighs. Place the chicken in a large resealable plastic bag or nonreactive container. Pour the 1 cup of marinade over the chicken, seal the bag (or cover the container), press or turn to evenly coat all pieces, and refrigerate for at least 1 hour or up to overnight.

Prepare to cook: When ready to cook, remove the chicken from the refrigerator. Discard any leftover marinade that remains in the bag or container (do not use it as a sauce unless you boil it). Preheat a large skillet over medium heat and add the reserved 1 tablespoon olive oil, tilting the pan to coat the bottom.

Cook the chicken in batches: Place enough chicken in the preheated skillet so pieces are not overcrowded. Cook, turning occasionally, until an instant-read thermometer inserted into the thickest part reads 165°F (about 10–15 minutes total depending on thickness). If you have more chicken, cook the remaining pieces in a second batch, adding the reserved oil again between batches if needed.

Rest and finish: Transfer cooked chicken to a large cutting board and let rest for at least 10 minutes. Slice or chop the chicken as desired and serve.