1. Preheat the oven to 200. 2. Cut the chicken breast crosswise into 1/4-inch-thick strips. Place the chicken in a small bowl. 3. Add 6 tablespoons of the lemon juice, 2 tablespoons of the coriander (if using), the cumin, cornstarch, salt and pepper and mix gently until the chicken is well coated; set aside. 4. Stack the tortillas and wrap them in foil. Place them in the oven to warm for 5 to 10 minutes. 5. Maenwhile make the sauce: In a small bowl, stir together the salsa and the remaining 2 tablespoons of lemon juice and 4 tablespoons of coriander (if using). 6. Shred the lettuce. 7. In a medium skillet, warm the oil over medium-high heat. Add the chicken and saute until white and firm, about 5 minutes. 8. Place the tortillas, chicken, shredded lettuce, sauce and sour cream in serving dishes. The diners can then assemble their own fajitas by topping a tortilla with some of each of the ingredients, ending with a dollop of sour cream, and then rolling it up. Time-Savers: Microwave Tip: Just before serving, stack the tortillas and wrap them in a damp paper towel. Cook at 100% for 1 minute. Do-Ahead: Since the fajitas are fine served at room temperature, much of the preparation can be done ahead of time. The sauce (Step 5) can be made and the chicken marinated in advance. Just be sure to cook the chicken and warm the tortillas as close to serving time as possible.