Puree first 5 ingredients (cilantro - chili powder) in blender (or food processor).
Place chicken in a glass baking dish and pour 1/3 of marinade over chicken, turning to coat all sides.
Arrange vegetables (chilies - onions) on a rimmed grilling pan or baking pan, pour 1/3 of marinade over vegetables. Season chicken and vegetables with salt and pepper if desired.
Grill chicken until cooked through (7 -10 minutes per side, depending on thickness of chicken). Grill vegetables until tender, turning often (about 15 minutes). Grill tortillas until charred (1 minute per side).
Transfer chicken to work surface and slice cross wise into strips.
Fill tortillas with chicken and vegetables and drizzle with remaining marinade.