Preheat the oven to 400 degrees. In a small bowl, mix together the fajita seasoning ingredients. On a sheet pan, add the chicken, bell pepper, and onion slices. Drizzle with olive oil, and sprinkle on the mixed fajita seasoning. Toss to coat. Roast in the oven for 30 minutes. Serve on warmed tortillas and with other desired toppings.