Heat taco shells as directed on package.
Meanwhile, in 10-inch nonstick skillet, heat 1 tablespoon of the oil over medium-high heat until hot. Cook chicken in oil about 6 minutes, stirring occasionally, until no longer pink in center. Remove chicken from skillet, and keep warm.
In same skillet, heat remaining 1 tablespoon oil until hot. Cook onion in oil about 4 minutes, stirring occasionally, until tender. Stir in garlic, jalapeño chili, cumin, chili powder and salt. Cook 1 minute, stirring constantly. Return chicken to skillet. Stir in tomatoes. Cook about 2 minutes, stirring occasionally, until hot.
To serve, place 1/4 cup shredded lettuce in each taco shell; divide chicken mixture among shells. Top with cheese and cilantro.