Pea and Watercress Purée

Makes about 3 cups, serving 6.

two 10-ounce packages frozen peas
2 bunches watercress, coarse stems discarded and the watercress rinsed
and spun dry (about 4 cups)
1/4 cup water
2 tablespoons unsalted butter

In a large saucepan combine the peas, the watercress, and the water and boil the mixture, covered, for 5 minutes, or until the peas and watercress are tender. Transfer the mixture to a food processor, add the butter and salt and pepper to taste, and puree the mixture.