Persian chicken with caramelized onion and spinach rice

This one-pot "Spinach Tah-chin" recipe is inspired by Yotam Ottolenghi's Chicken with caramelized onion and cardamom rice recipe. To prepare the dish, chicken thighs are first browned and then braised while layered in an aromatic rice, spinach, and kefir mixture. For the keen reader, Aanother Spinach Tah-chin recipe may be found here

4 chicken thighs
Salt and pepper to season
1 medium yellow onion, finely chopped
2 cinnamon sticks
1/4 TSP cardamon powder
1/2 TSP turmeric
Pinch of saffron
1 TBSP sugar or honey
2 cups rice
2~3 cups water(depending on your desired rice texture)
1 cup kefir (or buttermilk or diluted yogurt)
200 gr (7 Oz) fresh spinach leaves

Brown the thighs in a smoking hot non-stick pot. When thighs are golden brown, add the onion and lightly caramelize (dark caramelization would affect the desired yellow color of the final dish). To the chicken pot, add the spices, rice, kefir, and water and then the spinach, reserving a handful of spinach for a later application.   
Cook over medium heat until the surface liquid is evaporated, stir occasionally. lay the reserved spinach on top, lower the heat, lid the pot and cook for 40 minutes. Do not open the pan as the generated steam is crucial for the proper cooking of the rice.