Preheat oven to 375 degrees F. Spray a 13x9-inch baking dish with cooking spray.
In the prepared dish, stir together diced chicken, uncooked quinoa, bell peppers, onion, and diced tomatoes (with juices). Sprinkle with fajita seasoning and salt; toss to coat.
In a small bowl, whisk together water and taco sauce. Pour over quinoa mixture and stir to combine.
Cover dish with aluminum foil and bake for about 1 hour. Remove the cover and check to make sure that the quinoa and chicken are both cooked through.* If necessary, stir and return to the oven for a few more minutes (or until done). Serve immediately with optional toppings.
Note: you can prepare this dish ahead of time, cover, and refrigerate until ready to bake.