Combine curry powder, onion, and butter in a small plastic container. Combine cashews, raisins, and peas in small zip-top baggie.
Cook rice according to package directions; set aside and keep covered. Add curry mixture to medium pot. Heat over a low flame, stirring constantly until butter is melted and spices are aromatic. Add cashew/raisin/pea mixture and stir to coat. Add ¾ cup water and cook over medium heat until peas are soft and liquid simmers (about 5 to 7 minutes), then stir in coconut milk, vinegar, and sugar. Continue cooking until mixture is hot and thickens to a creamy consistency, like alfredo sauce. Serve over rice.