In a heavy saucepan, melt butter and cook onion with salt to taste over moderately low heat, stirring occasionally until softened, about five minutes. Add peas and stock and simmer, uncovered, until peas are tender, five to seven minutes. Remove pan from heat and place in a bowl of ice to stop peas from cooking.
In a blender, purée soup in batches until moderately smooth. Pour into one container. Whisk in cream and add salt and pepper to taste. Cover and chill soup until ready to serve.
For garnish, blend yogurt and curry powder thoroughly in a mixing bowl. Dollop onto cold soup. Garnish with fresh whole peas.