Preheat the grill to 400°F (200°C).
In a small bowl, mix Greek yogurt with harissa paste and season lightly with salt and pepper (reserve the remaining salt and pepper).
In a large bowl, add the chicken thighs, season with salt and pepper, and toss to coat. Pour in half of the harissa-yogurt sauce and toss again to coat evenly. Reserve the remaining sauce for serving.
In a small bowl, stir together 1 tablespoon of oil, minced garlic, and 1 teaspoon of parsley. Season with salt and pepper. Brush this garlic oil over the pita bread and set aside.
Grill the chicken for 6–8 minutes per side, until fully cooked, then slice into pieces.
When the chicken is almost done, place the pita on the grill and cook for 1–2 minutes on each side until lightly toasted.
In a large bowl, whisk together white wine vinegar, sugar, and 1 tablespoon of oil.
Add the spring mix, cucumber, and grape tomatoes. Season with salt and pepper and toss to combine.
To assemble, spread some of the reserved harissa sauce on the grilled pita, add sliced chicken, top with the salad, and sprinkle with the remaining parsley. Serve immediately.
