SPRING-PEA SAUCE

Steps:

• Prepare an ice-water bath; set aside. Bring a medium pot of water to a boil; add salt. Blanch peas and watercress until bright green, about 45 seconds. Immediately transfer with a slotted spoon to ice-water bath to stop the cooking.
• Drain peas and watercress, then puree in a blender until smooth, adding 4 to 5 tablespoons water to thin (mixture should be just thick enough to coat the back of a spoon). Pass puree through a fine sieve into a small saucepan. Place over low heat, and whisk in butter, 1 teaspoon at a time; whisk until emulsified. Season sauce with salt.