Cut the onions and sauté in a pan on low heat until they become soft, have a bright pink color to them, and have released all their water. Keep mixing and add spices. Spread the onions on the bread. Sprinkle sumac and nuts, and repeat.
Mix the spices together with the olive oil, then brush onto each chicken piece from all sides. Place onto a baking sheet lined with parchment paper, making sure the chicken is not tightly packed so it has enough room to crisp up.
Cover with aluminum foil, and bake for about 1 hour, then uncover and bake for an additional 5-10 minutes, so the skin gets crispy and develops a golden-brown color.
Start with 1 Naan or Taboon. Using a slotted spoon, take some onions and spread on top. Depending on the size of your bread, add enough to reach the edges but not so much that it piles up in the middle.
Serve each chicken on a bread piece topped for one person or layer multiple bread pieces on top of each other. Then, add the chicken on the final top layer.