In a bowl, combine the yogurt, olive oil, cubed chicken, garlic, shallots, paprika, oregano, chili flakes, and a large pinch each of salt and pepper. Let marinate for 15 minutes at room temperature or up to overnight in the refrigerator.Preheat the oven to 425° F. Arrange the chicken on a baking sheet. Bake 15 minutes, toss and bake another 5-10 minutes, or until cooked through. Switch the oven to broil. Broil 1-2 minutes, until the chicken chars on the edges.Meanwhile, combine the cucumbers, avocado, lemon, dill, salt, and pepper. To make the tahini. Combine all ingredients and 1/4 cup water in a blender and blend until smooth. If needed, add water to thin the sauce as desired. Season to taste with salt.To assemble, add lettuce to a bowl. Top with chicken, cucumber/avocado, feta cheese, and any other desired toppings. Add a few dollops of Tzaziki sauce. Drizzle over the tahini.