In a food processor, combine lime juice, oil, cilantro, scallion, garlic, salt, and red-pepper flakes; process until pureed. Place tenders into a bowl. Add the pureed lime mixture and stir to blend. Marinate at least 30 minutes. To cook, heat a large nonstick skillet over medium-high heat. Add the tenders, a few at a time. Quickly sear both sides until the chicken is cooked through, 2 to 3 minutes per side.
To make the fajitas, wrap the tortillas in foil and warm them in a 350 degrees F oven for 10 minutes. Separate the tortillas; place onto each a few tenders, a few slices of avocado, and a spoonful of salsa. Top with a spoonful of sour cream or plain yogurt, if desired. Roll up the tortillas and serve.