Delicious Chicken Salad with Yogurt Marinade and Pita Bread

For the Yogurt Marinade:
1 cup (100g) plain, low-fat yogurt
1 teaspoon lemon juice
2 tablespoons red wine vinegar
4 cloves garlic, minced
2 teaspoons ground cumin
1 teaspoon dried oregano
2 teaspoons sweet paprika
½ teaspoon ground cinnamon
Salt and freshly ground black pepper to taste
For the Chicken:
4 skinless chicken breasts, cut into small chunks or 8 boneless, skinless chicken thighs (trimmed of excess fat)
For the Salad:
2 small red onions, thinly sliced
4 tomatoes, deseeded and thinly sliced
1 teaspoon sumac (or a squeeze of lemon juice)
½ medium cucumber, grated
2 tablespoons chopped fresh dill
4 cups shredded lettuce (such as romaine or iceberg)
For the Tzatziki Sauce (optional):
Remaining grated cucumber (squeeze out excess water)
1 tablespoon red wine vinegar
1 clove garlic, minced
2 cups (200g) plain, low-fat yogurt
2 tablespoons chopped fresh dill
Salt and freshly ground black pepper to taste
For Serving:
4 whole-wheat pita breads, toasted