combine the cinnamon, cloves, coriander, and cumin in a mortar or coffee grinder and grind to a fine powder.
heat a large saute pan over med-high heat and add the oil. add the onions and saute, stirring frequently, for 10-15 minutes, until browned. add the spice mixture, garlic, ginger, chile, and salt and stir for 3-4 minutes. stir in the tomatoes and cook for 5 minutes, or until some of the liquid has evaporated.
place the chicken in the slow cooker and pour the onion mixture over the top. cover and cook on low for 3-8 hours, until the chicken is tender. at 3-4 hours the chicken will still be firm and hold its shape. at 6-8 hours, the meat will be falling off the bone.
in a small bowl, combine the saffron and half and half. let soak for 30 minutes. stir the saffron cream into the chicken 30 minutes before the end of cooking. season with salt to taste. transfer to a serving dish and garnish with the cilantro.