Place the onions on the bottom of the slow cooker. Sprinkle brown sugar over top.
Place the sliced red, orange, yellow, and green peppers over top.
Place the chicken over top in an even layer.
In a mixing bowl combine the Italian dressing, and all the spices. Whisk until combined. Pour over the ingredients in the slow cooker.
Turn slow cooker on low and cook for approximately 6 hours, or more, depending on how soft you like your peppers and onions.
Once the slow cooker is done cooking, give the ingredients a good stir and transfer to a bowl or platter. Drain any excess liquid.
Heat a frying pan over medium heat and place a tortilla inside, cook for 1 minute, flip and cook 1 minute more. Transfer tortilla to a platter. Repeat with as many tortillas as you need.
Top each tortilla with the fajita mixture. Pile each one as high as you like, then dig in.