Make marinade for chicken: In large bowl, combine 1/4 cup (60ml) vegetable oil, garlic, salt, Worcestershire sauce, chili powder, cumin, paprika, optional sugar and lime/lemon juice.Add chicken to the marinade, then cover and chill for about 20 minutes.
Cook the Vegetables: Heat a large skillet on high heat and add 1 Tablespoon of oil. Then add onions cook onions until almost soft. Then add bell peppers and minced jalapeños (optional). Sear the peppers and onion until slightly charred. Remove pepper and onion mixture to bowl. Put aside.
Cook the Chicken: In same pan, add the marinated chicken. DO NOT add the excess marinade juices because it will make the chicken less charred and crispy. But if you want a more wet fajita mixture, you can add the excess marinade.Cook chicken until done, then add pepper/onion mixture back into the skillet with the chicken. Cook for 1 more minute or until onions are hot and the mixture is combined well.
Serve with corn or flour tortilla, cheese, sour cream or fresh salsa.