Preheat your oven to 375°F (190°C). Season the chicken thighs with salt, pepper, 2 tablespoons of sumac, ground cumin, and ground coriander.
In a large skillet over medium heat, add the olive oil. Once hot, sear the chicken thighs until they are golden brown on both sides, about 5 minutes per side. Transfer the chicken to a baking dish and set aside.
In the same skillet, add the sliced onions and a pinch of salt. Cook over low heat, stirring occasionally, until the onions are deeply caramelized and sweet, about 30 minutes. Halfway through cooking, sprinkle the remaining sumac over the onions.
Once the onions are caramelized, layer them on top of the chicken thighs in the baking dish. Cover with aluminum foil and bake in the preheated oven for about 30 minutes, or until the chicken is fully cooked and tender.
While the chicken is baking, toast the pine nuts in a dry pan over medium heat until golden brown, and set aside.
To assemble, place the flatbreads on a baking sheet and warm in the oven for a few minutes. Remove the chicken from the oven and let it rest for a few minutes before placing on top of the flatbreads.
Spoon the caramelized onions over the chicken, sprinkle with toasted pine nuts and chopped parsley. Serve immediately.