Place the raw chicken in a mixing bowl along with 2 tablespoons of olive oil and the chili powder and sea salt. Use your hands to mix everything together so that the chicken is well-coated.
Heat the remaining 2 tablespoons of oil in a large cast iron skillet over high heat. While the skillet is heating, chop the onion, peppers, and garlic.
Add the sliced onion to the skillet and cook, stirring occasionally, until the sides begin to brown, about 2 to 3 minutes. Add the peppers and saute, stirring occasionally until peppers reach desired doneness, about 6 to 8 minutes. Add the garlic and continue cooking until fragrant, about 1 to 2 minutes.
While the peppers are cooking, heat a separate skillet over medium and add the chicken. Cook, untouched for 2 minutes. Flip the strips of chicken and continue cooking, flipping every 2 minutes or so, until chicken is completely cooked through, about 6 to 8 minutes total.
Once both the peppers and chicken are cooked, combine the two in one skillet. Serve chicken fajitas as desired - in tortillas with avocado, or over a bed of rice and beans.
*To make this recipe paleo, use grapeseed, almond, algae, or coconut oil for cooking.