For marinade, in blender, combine 3/4 of the cilantro along with scallions, jalapeño, olive oil, lime juice, lime zest and salt, and puree until uniform. Add splash of water if necessary to get blender moving.

Transfer mixture to zip-top bag or container and add chicken. Refrigerate and marinate 1-4 hours. Allow chicken to come to room temperature for 30 minutes before cooking.

Preheat grill or indoor grill pan to medium-high heat. When hot, place chicken on grill and discard remaining marinade. Cook until internal temperature reaches 165 degrees F, 6-7 minutes per side. Transfer to cutting board to rest 5 minutes.

Dice chicken and transfer to bowl. Add diced onion, reserved cilantro leaves and additional lime juice, if desired. Season to taste with salt and pepper. Portion chicken into tortillas or with taco shells and serve with side of guacamole.