Preheat the oven to 450 degrees and line a baking sheet with foil. Set aside 2 tbsp. cilantro leaves for the rice. Puree the remaining cilantro leaves and stems, the serrano, garlic, 1/4 cup water, the lemon juice, 2 tbsp. coconut, 1/2 tsp. salt and a few grinds of pepper in a blender until smooth, adding up to 2 tbsp. more water, if needed.
Toss the chicken thighs with half of the cilantro sauce on the prepared baking sheet; arrange skin-side up. Season generously with salt and pepper. Roast until the skin is crisp and the chicken is cooked through, about 25 minutes.
Meanwhile, heat the vegetable oil in a medium saucepan over medium heat. Add the curry powder, turmeric and a few grinds of pepper; cook, stirring until toasted, 30 seconds. Add the rice and cook, stirring, until well coated, 1-2 minutes. Add 1 1/2 cups water and 1/2 tsp. salt. Reduce the heat to low. Cover and cook until the liquid is absorbed and the rice is tender, about 15 minutes. Remove from the heat and let stand, covered, 5 minutes.
Fluff the rice with a fork. Stir in the cashews, the remaining 2 tbsp. coconut and the reserved 2 tbsp. cilantro. Serve with the chicken; top with remaining cilantro sauce.