Slice chicken thinly and place in a glass baking dish; mix well with garlic, oregano, and oil. Refrigerate for a day... Note, if grilling chicken on the BBQ do not slice before cooking, slice after grilling instead. Tzatziki Grate cucumber and let drain at least 5 min. Add to yogurt, garlic and oil. Adjust seasoning with salt and pepper. Best if made a day ahead Tahini In a blender, add tahini, lemon juice, salt, pepper and about 1/4 cup water. As ingredients blend, the paste will thicken to a peanut butter consistency. Keep adding water, a little at a time until it's a thinner consistency. I a glass bowl add sliced onion, parsley, and sumac, mix well; let sit at least 30 min. Until onion starts to soften and release some juices. Before cooking chicken, make sure all ingredients are prepared for assembly In a med frying pan on high heat sauté chicken make sure not to move too often so it browns. Cook 5 min or so. Brush pitas with oil, and place in 375 F oven to warm Remove and place on a sheet of foil Assembly; spread tzatziki on pita, add cheese, chicken, tomatoes, lettuce, and a generous portion of onion. Top with more tzatziki, and a small amount of tahini sauce. Wrap pita in foil like a burrito top with some hot peppers or serve on the side