Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.
To make the fajita seasoning, whisk the chili powder, cumin, paprika, coriander, salt, and pepper in a small bowl, and set aside.
Add the chicken to the center of the sheet pan. Put the onions on one side and bell peppers on the other. Alternatively, you can spread the bell peppers and red onion onto the baking sheet first, then add the chicken on top.
Sprinkle with fajita seasoning and drizzle with avocado oil. Toss everything to evenly coat. Spread into an even layer..
Roast in preheated oven for 18-25 minutes (Mine took 22 minutes). Toss once halfway through cooking. Vegetables should be tender and chicken cooked through.
Warm the tortillas in the oven. Fill the tortillas with chicken and veggies and sprinkle with toppings like cheese, salsa, guacamole, hot sauce, and sour cream.