Put a baking sheet in the oven and preheat the oven to 400 degrees F.
In a large bowl, whisk the oil, vinegar, onion powder, oregano and garlic powder; season with salt and pepper. Add the chicken, tomatoes and onion and toss to coat.
Carefully remove the baking sheet from the oven and evenly spread the chicken mixture on it.
Bake for 10 minutes, then add the pita around the edge of the baking sheet and bake until the chicken is cooked through, another 5 minutes. (If desired, you can also broil the mixture for a couple of minutes more for more color.)
Serve with the Tzatziki Dip.
Combine the yogurt, cucumber, vinegar, mint, salt and garlic in a small bowl. Refrigerate until serving.