Crumble the saffron threads into the hot water and let steep 10 minutes.
In a bowl, combine the chicken thighs and drumsticks with the kosher salt, black pepper, ground ginger, ground turmeric, and the garlic; rub to coat well and let stand 30 minutes at room temperature.
Warm the extra-virgin olive oil in a tagine or heavy pot over medium heat; add the yellow onion and cook, stirring, until translucent and lightly golden, 8–10 minutes.
Add the marinated pieces and any juices to the pot; cook over medium heat, turning once, until lightly colored on both sides, 6–8 minutes total.
Stir in the water, the saffron infusion, and the smen; bring to a gentle simmer, then cover, reduce heat to low, and cook until the pieces are very tender, 35–45 minutes, turning once halfway.
Uncover; add the preserved lemon and the green olives, and simmer until the sauce reduces to a glossy consistency that coats the pieces, 10–12 minutes.
Stir in the cilantro leaves and the flat-leaf parsley; remove from the heat and rest 5 minutes.