Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Dice the chicken breast, bell pepper, zucchini, and red onion into bite-sized pieces.
Place the chicken and vegetables on the baking sheet, drizzle with olive oil, and season with oregano, garlic powder, salt, and pepper.
Toss everything together until evenly coated, then spread out in a single layer.
Roast for 18-20 minutes, or until the chicken is cooked through and the vegetables are slightly charred.
While the filling roasts, whisk together the Greek yogurt and lemon juice in a small bowl.
Slightly warm the pitta bread in the oven for the last 2 minutes of cooking.
Slice the pitta in half, stuff with the roasted chicken and vegetable mixture, and finish with a drizzle of the lemon-yogurt sauce.