Place onion, olive oil, lime juice, vinegar, cumin, garlic, red peppers, sea salt, and black pepper in a blender. Blend until smooth. Place chicken in a container or resealable plastic bag and pour marinade on top; toss to coat. Marinate in the refrigerator for 2 to 24 hours.

Preheat an outdoor grill for medium-high heat and lightly oil the grate.

Grill chicken until no longer pink in the centers, 6 to 7 minutes per side, depending on thickness. An instant-read thermometer inserted into the centers should read at least 165 degrees F (74 degrees C).