In the bottom of a tagine, pressure cooker or large heavy pot, mix the chicken with the sliced onion, ginger, salt, pepper, cinnamon, ras el hanout, saffron, optional smen, and olive oil. Over medium heat, brown the chicken pieces, turning occasionally, for about 15 minutes.
Add 6 cups of water to the pot, cover, and bring to a boil. Reduce the heat to medium and continue simmering until the chicken is cooked but not too tender to break apart — about 30 minutes depending on the size of the chicken cuts.
The broth should cover the chicken, allowing ample sauce to cook the rice in and add to the Tfaya mix. Check the level of the broth and taste test it for salt, it should be slightly on the salty side which will adjust as the rice cooks in it and broadens.
Take out the chicken pieces from the tagine stock, place them on a lined oven tray and place them in the oven to roast until golden and crisp.
Add the washed and soaked rice and cook as per rice packet instructions replacing the water with the chicken tagine stock. Add more water if needed but check and adjust the seasoning to taste. (The exact time of cooking will depend on the type of rice you use.)