Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper.
Slice the bell peppers into strips and cut the onion into wedges.
Place the sliced bell peppers, onion wedges, and shredded rotisserie chicken on the sheet pan. Drizzle with olive oil and sprinkle taco seasoning over the vegetables. Toss to coat evenly.
Roast in the preheated oven for about 25 minutes or until the vegetables are tender.
While the fajitas are in the oven, warm some tortillas. Serve the fajita mixture on tortillas. Add any toppings you want!