The late, great Gascogne chef Jean-Louis taught me this simple, intense sauce, which highlights all the flavor of the peppery greens. I have published many of his recipes in my other books. This is one I found in an old notebook, and it is, perhaps, the easiest recipe in his repertoire.
Salt and freshly ground pepper
About 1teaspoonlemon juice
3tablespoonsglace de homard or glace de poisson (see Note)
Rinse the watercress carefully; pick over and remove all tough stalks. There will be about 2cups leaves and tender stems. Boil in salted water for 10 minutes. Drain and rinse under cool running water to refresh; drain again. Squeeze the watercress with your hands to remove as much water as possible. Place the watercress in an electric blender, add ½cup of the heavy cream, and puree until smooth. Season with ½teaspoon salt, ¼teaspoon pepper, and ½teaspoon lemon juice. Blend to combine.
In a heavy saucepan, melt the glace de homard over low heat, whisking. Stir in the remaining ¼cup cream, then the watercress puree. Correct the seasoning, adding lemon juice to taste. Keep hot.