Step 1

Place chipotle chile pepper and ancho chile pepper in a bowl; pour in water. Cover bowl and let sit until peppers are softened, at least 10 to 12 hours. Drain water and remove seeds from peppers.

Step 2

Blend chile peppers, red onion, garlic, sea salt, cumin, oregano, and black pepper in a blender until a coarse paste forms; add olive oil and blend until marinade is smooth.

Step 3

Pound chicken thighs flat with a meat mallet and trim excess skin. Place chicken in a resealable plastic bag and add marinade. Turn chicken several times to coat. Seal bag and marinate at least 8 hours.

Step 4

Preheat an outdoor grill with top and bottom plates for medium-high heat. Remove chicken from bag and discard marinade.

Step 5

Cook chicken on the preheated grill until no longer pink in the center and the juices run clear, 5 to 10 minutes. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C). Cut into strips.