Prep the oven and pan
Preheat the oven to 425°F. Line a large sheet pan with parchment paper or lightly grease it for easier cleanup.
Season the chicken and vegetables
In a large bowl, toss the chicken strips, bell peppers, and red onion with olive oil, garlic powder, paprika, oregano, salt, and black pepper until everything is well coated.
Roast
Spread the chicken and vegetables out on the prepared sheet pan in an even layer. Roast for 20 to 25 minutes, stirring once halfway through, until the chicken is fully cooked and the vegetables are tender with lightly browned edges.
Make the herby ranch slaw
While the chicken cooks, stir together the Greek yogurt, mayonnaise, lemon juice, dill, parsley, chives, garlic powder, salt, and pepper in a medium bowl. Fold in the shredded cabbage mix until everything is coated. Chill until ready to serve.
Warm the pita
Warm the pita breads in the oven for a couple of minutes, or heat them in a dry skillet until soft and pliable.
Assemble
Fill each pita with chopped lettuce, roasted chicken and vegetables, cucumber, tomatoes, and a generous scoop of herby ranch slaw.
Serve
Serve right away while the chicken is warm and the slaw is cool and crisp.