Spring Pea Sauce

Directions

1. 1
   Prepare an ice-water bath; set aside.
2. 2
   Bring a medium pot of water to a boil; add salt.
3. 3
   Blanch the peas and watercress until bright green, about 45 seconds.
4. 4
   Immediately transfer with a slotted spoon to the ice-water bath to stop the cooking.
5. 5
   Drain the 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).
6. 6
   Pass the puree through a fine sieve into a small saucepan.
7. 7
   Place over low heat, and whisk in the butter, 1 teaspoon at a time; whisk until emulsified.
8. 8
   Season the sauce with salt.