Slice chicken breasts into 1/2-inch wide strips and place them in a large bowl.
Sprinkle the lime zest and juice, cumin, paprika, and chili powder over the chicken. Mix together, and allow to marinate for 30 minutes.
Thinly slice the onions. Core, seed, and slice the bell peppers into thin, uniform strips.
Heat the oil in a large skillet. Stir-fry the marinated chicken for 5 - 6 minutes.
Add the onions and bell peppers and cook for an additional 3 - 4 minutes, or until the chicken is thoroughly cooked and the vegetables are soft and tender. Stir in the garlic and cook for one additional minute until fragrant.
Place the chicken fajitas in a large serving bowl.
Serve with heated tortillas and your choice of toppings.
Notes
Wrap tortillas tightly in foil and heat in a 400 degree oven for 10 minutes to warm them before serving.
Drizzle lime juice over the fajitas for even more flavor.