1. Drop the jalapeno down the chute of a food processor with the motor running and puree. (If using a fresh jalapeno, stem and seed it first.) Stop the motor and scrape down the sides of the bowl with a rubber spatula. Peel and quarter the onion. Add the onion and cilantro leaves to the processor and pulse a few times.
2. Place each mango, narrow side down, on a cutting surface. Slice through the mango as close to the pit as possible on one side, then repeat on the other side. With a teaspoon, scoop out the flesh from the two thick slices and cut each slice into 4 pieces. Juice the lime. Add the mango and lime juice to the processor and pulse just until the salsa is fully combined but still chunky.
3. Put the oil in a wok or large, heavy skillet over medium-high heat. Cut the chicken into strips about .5 inches wide and 2 to 3 inches long. Season the strips with salt and pepper. Raise the heat to high and add the chicken. Cut the top from the bell pepper. Stand it upright and cut down inside the four walls, separating them from the center core and seeds. Then cut the walls into thin strips. Add the strips to the chicken and cook, stirring periodically, for 5 minutes or until the chicken is just done. The chicken should feel firm. (Cut through the center of one strip to check if you’re not sure.)
4. While the chicken cooks, spread the tortillas on a microwave-safe plate and cover with a paper towel. Cook in a microwave oven on high power for 20 seconds. Put the chicken and peppers on a small serving platter. Put the salsa in a small bowl. Bring the food to the table for diners to make their own fajita. To assemble, put one-eighth of the chicken mixture on each tortilla, top with a tablespoon or more of salsa, and fold the tortilla over. Allow two fajitas per person.