Combine the flour, egg, low-fat quark, and oatmeal in a bowl. Optionally stir in sesame.
Spread the mixture onto a baking sheet into a single square.
Bake at 200 °C / 390 °F for 25 minutes.
While the flatbread bakes, cut the lettuce into small strips, the cucumber into slices, the onion into rings, and the tomato into slices. If using feta cheese, crumble it.
Cook the chicken breast fillet in a pan over medium-high heat until the internal temperature reaches 74 °C / 165 °F.
Mix the yogurt with spices to taste.
Once the flatbread is cooked, fill it with the vegetables and chicken, pour the yogurt sauce over it, and fold it up to serve.