Preheat the broiler. Put chicken breasts in a shallow nonaluminum dish and sprinkle with lime juice, salt and pepper. Let marinate at room temperature for 10 minutes.
Whisk together yogurt, sour cream, cilantro, jalapeno and cumin in a small bowl. Season with salt and set aside.
Place the chicken and onions on a lightly oiled baking sheet and broil for 3 to 4 minutes. Turn the chicken and stir onion. Broil until the chicken is no longer pink inside, 3 to 4 minutes.
Meanwhile, wrap tortillas in foil and place on the rack below the baking sheet.
To assemble fajitas, thinly slice the chicken. Fill each tortilla with the chicken, onions, lettuce and tomato slices. Top with the reserved yogurt mixture and roll up the tortilla and filling. Serve immediately.
To heat tortillas:
Microwave: Wrap in barely damp paper towels and microwave on high for 30 to 45 seconds.
Oven: Wrap in foil and bake at 300ºF until steaming, about 5 minutes.
Steamer: Wrap a stack of 12 tortillas in a clean, heavy towel, place in a steamer over 1/2 inch of water, cover and bring to a boil over medium-high heat. When steam is released, time 1 minute, then remove from the heat and let stand 15 to 20 minutes. Held in a very low oven, the tortillas will stay warm and moist for an hour or more.
210 calories;3 g fat(1 g sat); 4 g fiber; 29 g carbohydrates; 17 g protein; 19 mcg folate; 34 mg cholesterol; 3 g sugars; 0 g added sugars; 986 IU vitamin A; 11 mg vitamin C; 86 mg calcium; 2 mg iron; 248 mg sodium; 281 mg potassium