02Working in batches, purée; soup in a blender until smooth (use caution when blending hot liquids) and transfer to a bowl. Return puré to cleaned pot and thin with water if desired; simmer 3 minutes. Season with salt.
03Pulse mint, parsley, and scallion in a food processor until finely chopped. With motor running, add oil in a stream, then add water and salt, blending until incorporated.
04Swirl 1 tablespoon pistou into each bowl of soup.