Preheat your oven to 220°C (425°F). Line a large rimmed baking sheet with parchment paper or aluminum foil for easier cleanup. This step is crucial for ensuring even cooking and preventing sticking.

Prepare the vegetables: Peel and thinly slice the onions into strips (about 0.5cm / 0.2 inch thick). Seed and thinly slice the bell peppers into strips of similar thickness. Ensure all vegetable strips are roughly the same size for even cooking.

Prepare the chicken: If not already cut, slice the chicken breasts or thighs into thin strips, approximately 1cm (0.4 inch) thick. Ensure the strips are not too thick, as this will affect cooking time and caramelization.

Make the fajita seasoning: In a small bowl, whisk together the chili powder, cumin, paprika, garlic powder, dried oregano, salt, and black pepper. This blend creates the signature smoky and slightly spicy flavor of fajitas.

Combine ingredients for roasting: In a large bowl, combine the sliced chicken, sliced onions, and sliced bell peppers. Drizzle with the olive oil (60ml / 4 tbsp) and sprinkle with the prepared fajita seasoning. Toss everything thoroughly to ensure all ingredients are evenly coated with oil and spices. Make sure no ingredients are clumped together.

Arrange on the baking sheet: Spread the seasoned chicken and vegetable mixture in a single layer onto the prepared baking sheet. Avoid overcrowding the pan; if necessary, use two baking sheets. Overcrowding will cause the ingredients to steam rather than roast and caramelize.

Roast the fajitas: Place the baking sheet in the preheated oven at 220°C (425°F). Roast for 20-25 minutes, or until the chicken is cooked through and no longer pink in the center, and the vegetables are tender and slightly caramelized around the edges. For more caramelized vegetables, you can broil for the last 1-2 minutes, watching very carefully to prevent burning.

Warm the tortillas: While the fajitas are roasting, warm the tortillas. You can do this by wrapping them in a clean kitchen towel and microwaving for 30-60 seconds, or by warming them briefly in a dry skillet over medium heat for about 15-30 seconds per side, or directly over a low gas flame for a few seconds per side until pliable.

Serve: Carefully remove the baking sheet from the oven. Squeeze fresh lime wedges (from 1 medium lime) over the roasted chicken and vegetables. Serve the hot fajita mixture immediately with the warmed tortillas and your favorite toppings such as shredded lettuce, salsa, sour cream, guacamole, or shredded cheese.