Peel the shrimp and pat dry with paper towels. Cut onions in thin slices and cook in melted butter or margerine until limp. Toss in shrimp, turmeric, coriander and cook, uncovered, over moderate heat for 10 minutes. Stir frequently. Now mix in chili powder, salt, yoghurt and cook to a boil. Cover and cook gently for 10 minutes more. Remove cover and continue cooking over a low heat until most of the sauce disappears. Takes about 5 minutes. Serves 4 to 5 people.