Special equipment: 8 by 12 by 4-inch aluminum tray
In a large bowl, combine yogurt with garlic, onion, vinegar, parsley, oregano, salt, cumin pepper, red chili flakes, and sage. Add chicken and marinate in the refrigerator for 12 to 24 hours.
Preheat oven to 350 degrees F with oven rack in center.
Fit a 9 by 13-inch baking sheet with a cooling rack. Remove chicken from marinade, taking off excess marinade. Place meat randomly on top of each other on the aluminum foil tray, creating 1-inch of meat. Add spinach, spreading evenly, and repeat with another 1-inch layer of meat. Add a layer of red bell peppers, and finish by covering with another inch of meat.
Press layers firmly together. Remove foil tray with chicken to the baking sheet. Poke holes in bottom of foil to allow excess liquid to drain. Bake in center of oven
for 45 minutes until chicken is firm and holds together. Cut corners of tray and push the sides of foil down, to expose meat to brown. Continue to cook for another 30 minutes or until the meat registers an internal temperature of 165 degrees F. using an instant-read thermometer.
Let rest for 10 minutes. Remove Schawarma to a cutting board, discarding foil. Slice into 1/2-inch thick pieces.
Serve sandwiched in flatbread and drizzled with Tzatziki Sauce.
In a medium bowl, combine flour and salt. Using a wooden spoon, slowly stir in water, until dough forms. Lightly flour a clean board and knead dough until smooth, about 10 minutes. Rest dough in bowl, covered with damp towel for 20 minutes.
Preheat a griddle over medium heat.
Divide dough into 8 equal balls. Using a rolling pin, roll balls out on floured board to 7 inches in diameter, about 1/4-inch thick. Place the rolled out dough onto griddle. Flip when raised bumps appear on the surface of the dough, about 1 to 2 minutes. Cook second side until it turns golden. Do not cook too long or flatbread will become brittle. Cover flatbread with a towel up to 20 minutes before serving.