Middle Eastern Chicken Shawarma

Name: Middle Eastern Chicken Shawarma
Ingredients: For the Marinade; 1 cup yogurt (plain); 1/4 cup vinegar; 2 cloves garlic (crushed); 1 teaspoon pepper; 1/2 teaspoon salt; 2 cardamom pods; 1 teaspoon allspice; 2 tablespoons lemon juice (from 1 lemon; For the Chicken; 1 1/2 pounds chicken thighs or breasts (boneless, skinless, thinly cut); For the Sauce; 1 cup tahini; 1/4 cup lemon juice; 2 tablespoons yogurt; For the Sandwich; 1 onion (thinly sliced); 1 to 2 tomatoes (thinly sliced); 1 cucumber (thinly sliced); 1/2 teaspoon sumac; 1/2 cup fresh parsley (finely chopped); Optional: pickle slices; 8 small pita breads (4 large pita breads)