Melt the butter in a small saucepan. Add 1 tsp curry and turmeric 1 tsp. Mix the two spices, turning 1-2 times the pot. Put then the coconut milk, stir well and leave on low heat until thickened slightly. Serve sauce warm in a suitable container for sauce. Before serving, add some chopped lemongrass. The sauce is suitable for different types of fish and crustaceans.