Combine the coconut milk, ground turmeric, ground cinnamon, onion powder (if using), red pepper flakes (if using), and sea salt in a blender and blend until smooth.
Place the chicken thighs in a sealable container or zip-top bag and pour in 1/2 cup of the coconut-turmeric mixture. Marinate at least 15 minutes or up to 12 hours in the refrigerator.
Rinse the rice and transfer to a bowl or measuring cup. Cover with at least 2 inches of water and soak for at least 15 minutes or up to 12 hours. Drain and set aside.
Heat a large, deep skillet or Dutch oven over medium heat. Add the marinated chicken and sear until browned, about 3 minutes per side.
Add the soaked rice, the remaining coconut milk mixture, and 1 cup water to the skillet; stir to combine and bring to a full boil.
Stir in the saffron threads, frozen peas, and raisins, then return to a boil.
Reduce heat to low, cover, and simmer for 10 minutes.
Remove the skillet from heat, keep covered, and let rest for 10 minutes. Taste and adjust seasoning with more sea salt and/or a squeeze of lemon if desired, then serve with lemon slices.