Step 1: Slice the chicken breasts or thighs into thin, uniform strips (about 1/4 to 1/2 inch thick). Slice the bell peppers (red, yellow, orange recommended) and onion into similar-sized strips.
Step 2: In a medium bowl, combine the chicken with 2 tablespoons of olive oil and the fajita seasoning blend (chili powder, cumin, smoked paprika, garlic powder, onion powder, oregano, salt, and black pepper, plus optional cayenne). Toss until the chicken is thoroughly coated. Let marinate for at least 15-20 minutes at room temperature, or up to 4 hours in the refrigerator.
Step 3: Heat 1 tablespoon of olive oil in a large cast-iron skillet or heavy-bottomed pan over medium-high heat until shimmering.
Step 4: Add the sliced onions to the hot skillet and cook for 3-4 minutes, stirring occasionally, until they start to soften and become translucent.
Step 5: Add the sliced bell peppers to the skillet with the onions. Cook for another 5-7 minutes, stirring occasionally, until they are tender-crisp. Remove the cooked onions and peppers from the skillet and set aside.
Step 6: Add the remaining 1 tablespoon of olive oil to the same skillet. Increase the heat to high. Once very hot, add the marinated chicken in a single layer, ensuring not to overcrowd the pan. Cook the chicken for 3-5 minutes per side, or until beautifully browned and cooked through. Cook in two batches if needed.
Step 7: Once the chicken is cooked, return the reserved onions and peppers to the skillet with the chicken. Toss everything together for 1-2 minutes to combine and heat through. Squeeze fresh lime juice over the mixture just before serving.
Step 8: Serve immediately with warm tortillas and your favorite fajita toppings such as sour cream, guacamole, salsa, and shredded cheese.