Preheat your oven to 375°F (190°C).

In a large skillet or frying pan, heat 2 tablespoons of olive oil over medium heat. Add the sliced onions and cook until they are caramelized and golden brown, stirring occasionally. This may take about 15-20 minutes. Set aside.

In the same skillet, heat another 2 tablespoons of olive oil. Season the chicken with salt and pepper, then add it to the skillet. Cook the chicken until it is browned and cooked through, about 5-7 minutes per side.

In a small bowl, mix together the sumac, cinnamon, and allspice. Sprinkle the spice mixture evenly over the chicken, making sure to coat both sides. Cook for another minute or two to allow the spices to release their flavors.

Place the flatbreads on a baking sheet and brush them with a bit of olive oil. Warm them in the preheated oven for a few minutes until they become slightly crispy.

To assemble, place one flatbread on a serving platter or individual plates. Spread a generous amount of caramelized onions over the bread, then top with a few pieces of the spiced chicken. Repeat this layering process with the remaining flatbreads, onions, and chicken.

Once assembled, return the Musakhan to the oven and bake for about 10 minutes, or until everything is heated through.

Serve the Musakhan warm, with a side of plain yogurt for dipping. Enjoy!