Heat 2 to 3 tablespoons olive oil in a large skillet over medium-high heat until hot but not smoking.

Add the 1 to 1.25 pounds sliced chicken breasts in a single layer (work in batches if necessary). Cook 4 to 5 minutes, stirring and flipping intermittently, until the strips are lightly browned and no pink remains. Remove the chicken from the skillet and set aside, leaving the oil and cooking juices in the pan.

If the skillet looks dry, add a little more olive oil. Add the thinly sliced Vidalia onion and sauté over medium heat for about 5 to 7 minutes, stirring occasionally, until the onion softens and begins to caramelize.

Add the red and yellow bell pepper strips to the skillet. Sauté over medium heat for about 4 to 5 minutes, stirring occasionally, until the peppers soften but still have some bite.

Return the cooked chicken to the skillet with the onions and peppers.

Pour in 1/4 cup water and sprinkle in the one 1-ounce packet Old El Paso Fajita Seasoning Mix. Stir well to combine, then cook for about 2 to 3 minutes, stirring occasionally, until the chicken is heated through and the seasoning coats the mixture.

Warm the Old El Paso flour tortillas if desired: heat a dry skillet over medium and warm each tortilla about 15 to 30 seconds per side.

Spoon the chicken and pepper mixture into the tortillas and serve immediately. Top with guacamole, sour cream, cheese, lime juice, or your favorite toppings, if desired. Fajitas are best served warm.