Preheat the oven to 425 degrees. Mix the red and green peppers, onion and chicken tenders in a large bowl with the olive oil and taco seasoning. Completely coat the chicken, peppers and onions. Transfer to a rimmed baking sheet that has been sprayed with cooking spray. Cook for 30 minutes, stirring the mixture a few times throughout cooking. Assemble fajitas with avocado, sour cream, salsa and any other toppings you desire. Serve and enjoy!