Preheat the oven to 450 degrees F and line a baking sheet with foil. Set aside 2 tablespoons cilantro leaves for the rice. Puree the remaining cilantro leaves and stems, the serrano, garlic, 1/4 cup water, the lemon juice, 2 tablespoons coconut, 1/2 teaspoon salt and a few grinds of pepper in a blender until smooth, adding up to 2 tablespoons 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 to 2 minutes. Add 1 1/2 cups water and 1/2 teaspoon 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 tablespoons coconut and the reserved 2 tablespoons cilantro. Serve with the chicken; top with the remaining cilantro sauce.