Cut each chicken breast lengthwise in 3 strips; cut crosswise into thin pieces (diced is also good).
If using the Homemade Fajita Seasoning add 2 tablespoons of oil in a small bowl and combine well. OR: Combine the chili powder, cumin, oregano, garlic powder with 2 tbsp of the oil in a small bowl.
Toss the seasoning mix with the chicken, coating very well. Leave the chicken to marinate this way while you are chopping the vegetables or refrigerate and let it marinate for a couple of hours.
Heat the 1 tablespoon of oil in a large skillet over medium high heat.
Add the chicken and cook 7 to 8 minutes or until no longer pink in the center, stirring frequently. Let the chicken brown up nicely before turning it.
Stir in the bell peppers and onion. Cook the vegetables for 5 to 7 minutes or until the vegetables are tender.