Preparation Finely chop onion. In a 4-quart heavy saucepan cook onion in butter with salt to taste over moderately low heat, stirring occasionally, until softened. Add peas and 3 cups stock and simmer, uncovered, until peas are tender, 5 to 7 minutes. Stir in mint and remaining 2 cups stock and remove pan from heat. In a blender purée soup in batches until very smooth (use caution when blending hot liquids), forcing each batch through a sieve into a large bowl. (Discard solids in sieve between batches.) Whisk in cream (or cream-yogurt mixture) and salt and pepper to taste. If serving soup cold, chill, covered. If serving soup hot, reheat but do not let boil.
Preparation Finely chop onion. In a 4-quart heavy saucepan cook onion in butter with salt to taste over moderately low heat, stirring occasionally, until softened. Add peas and 3 cups stock and simmer, uncovered, until peas are tender, 5 to 7 minutes. Stir in mint and remaining 2 cups stock and remove pan from heat. In a blender purée soup in batches until very smooth (use caution when blending hot liquids), forcing each batch through a sieve into a large bowl. (Discard solids in sieve between batches.) Whisk in cream (or cream-yogurt mixture) and salt and pepper to taste. If serving soup cold, chill, covered. If serving soup hot, reheat but do not let boil.