Combine onions, ginger, chili pepper, and garlic into a mortar and pestle.Pound into a smooth paste.Heat oil a pot. Sauté prepared paste until aromatic.Stir in the beef. When brown, add coconut milk and lemon grass.Cook over moderate heat until beef is tender.Stir in the sour cream, turmeric, salt and pepper to taste.Continue to cook until sauce thickens.Stir in the butter and remove from heat.