Preheat oven to 400°F on convection (or 425°F regular bake)
Line a large baking sheet with heavy duty aluminum foil.
In a large bowl whisk together 3 tablespoons of oil, 1 ¼ tsp salt, the cumin, coriander, turmeric, cayenne, cinnamon and garlic.
Trim chicken thighs with kitchen shears and chop chicken into medium sized or bite sized pieces. Stir into the spices. Arrange chicken in a single layer on the prepared baking sheet.
In the same bowl (no need to rinse) add the remaining 1 tablespoon of oil, ¼ salt and the sliced onions. Stir to combine and spread out on top of and around the chicken.
Roast for 20 minutes, stir and bake for another 5-10 min until the chicken is cooked through and onions are beginning to caramelize.
While the chicken cooks, make the sauce. In a small bowl whisk together yogurt, mayonnaise, lemon juice, garlic and salt. Taste and season if necessary.
When the chicken is almost done pop the flatbread onto another rack in the oven for a couple of minutes.
Slice the chicken if needed and stir together in a bowl with the onions and juices. Serve with warmed flatbread, sauce and chopped veggies.