Preheat the oven to 400°F (200C)
In a large skillet, heat the olive oil over medium-high heat. Add the sliced onions and sauté until they are soft and translucent.
Add the chicken thighs to the skillet and sauté for a few minutes on each side until browned.
Add enough water to the skillet to cover the chicken. Add the sumac, salt, and pepper to taste.
Bring the water to a boil, reduce the heat to low and cover the skillet. Simmer for 30-40 minutes or until the chicken is cooked through and tender.
Remove the chicken from the skillet and set aside. Allow the cooking liquid to cool.
Combine the yoghurt, tahini, minced garlic, chopped parsley, chopped mint, salt, and pepper in a separate mixing bowl.
Heat a bit of olive oil over medium heat in a small skillet. Add thetorn Lebanese bread and sauté until crispy and golden brown.
To assemble the Mousakhan Fatteh, layer the toasted Lebanese bread in an oven proof dish.
Shred the cooked chicken with a fork or hand and mix it with the cooked onions.
Place the chicken and onion mixture on top of the Lebanese bread.
Spoon the yoghurt mixture on top of the chicken and spread it evenly.
Pour the cooled cooking liquid from the skillet over the yoghurt and chicken.
Bake in the oven for 10-15 minutes, or until the yoghurt mixture is heated through and bubbly.
Garnish with the toasted pine nuts and chopped herbs.
Serve immediately while the Lebanese bread is still crispy and the yogurt mixture is hot.