Combine first 4 ingredients in a large zip-top plastic bag; seal and marinate in refrigerator for 20 minutes.
Prepare grill or broiler.
Drop the garlic and jalapeño through food chute with food processor on, and process until minced. Add cilantro leaves; process until finely minced. Heat oil in a small saucepan over medium heat until hot. Add flour; cook 1 minute, stirring constantly with a whisk. Add cilantro mixture, broth, and salt; cook 4 minutes or until slightly thick, stirring occasionally. Remove from heat; keep warm.
Remove the chicken and bell pepper pieces from bag, and reserve marinade. Place chicken and bell pepper pieces on grill rack or broiler pan coated with cooking spray. Cook 5 minutes on each side or until the chicken is done, basting with marinade. Divide cilantro sauce evenly among 4 plates. Arrange the chicken and bell pepper pieces on top of the sauce. Garnish with cilantro sprigs, if desired.