In a large bowl, whisk together 1/2 cup oil, lime juice, cumin, and red pepper flakes. Season chicken with salt and pepper, then add to bowl and toss to coat. Let marinate in the fridge at least 30 minutes and up to 2 hours.
When ready to cook, heat remaining tablespoon oil in a large skillet over medium heat. Add chicken and cook until golden and cooked through, 8 minutes per side. Let rest 10 minutes, then slice into strips.
Add bell peppers and onion to skillet and cook until soft, 5 minutes. Add chicken and toss until combined. Serve with tortillas.