Toss chicken with curry powder, salt, and pepper in a medium bowl. Heat 2 Tbsp. oil in a large skillet (at least 12" in diameter) over medium-high. Add onion and cook, stirring, until softened, about 2 minutes. Purée coconut milk, ginger, and garlic in a blender until very smooth. Add chicken and coconut milk mixture to skillet and cook, tossing occasionally, until chicken is cooked through and sauce has thickened, 7-10 minutes. Meanwhile, if using, heat 2 tsp. oil in a small skillet over medium. Add cashews and mustard seeds and cook, stirring, until fragrant and lightly browned, 2–3 minutes. Immediately transfer mixture including the oil to a small bowl. Fold spinach into chicken mixture and cook until wilted, about 1 minute. Divide among 4 bowls. Top with cilantro and cashew mixture, if using. Serve with rice or naan alongside. Editor's note: This article has been updated as a part of our archive repair project.