Start by gathering all your marinade ingredients. In a blender or food processor, combine the coarsely chopped red onion, garlic, adobo sauce, ancho chile powder, olive oil, ground cumin, and dried oregano. Add a pinch of salt and freshly ground black pepper. Blend until smooth and well combined.
In a large mixing bowl, place the boneless, skinless chicken. Pour the marinade over the chicken, ensuring each piece is evenly coated. Cover with plastic wrap or a lid and refrigerate for at least 1 hour, but preferably overnight.
When you're ready to cook, heat a skillet or grill over medium-high heat. Add a little oil to the surface to prevent sticking.
Remove the chicken from the marinade, allowing any excess to drip off. Place the chicken on the hot skillet or grill. Cook for about 5-7 minutes on each side, or until the chicken is cooked through and has a beautiful char. Use a meat thermometer to check that the internal temperature reaches 165°F (75°C).
Once cooked, transfer the chicken to a cutting board and let it rest for a few minutes. Then, slice it into strips or bite-sized pieces. Serve it over rice, in tacos, or in a burrito.
