Lemony Chicken Souvlaki With Celery and Feta

Name: Lemony Chicken Souvlaki With Celery and Feta
Ingredients: 4 stalks celery, thinly sliced; 1/8 cup chopped fresh mint; 1/2 to 1 teaspoon crushed red pepper; 4 tablespoons olive oil; 2 teaspoons grated lemon zest, plus 2 tablespoons fresh lemon juice and 2 lemons, halved; kosher salt and black pepper; 6 boneless, skinless chicken thighs (about 1 3/4 pounds), cut into 1 1/2-inch pieces; 1/2 small red onion, cut into 1 1/2-inch pieces; 4 pocketless pitas or flat breads, warmed; 2 ounces Feta, crumbled (1/2 cup)