From Michel Roux: Sauces, by Michel Roux ($24.95, hardcover with color photographs, Rizzoli New York).
Makes about 3/4 cup.
1 small potato, about 2 ounces
1 1/2 cups snipped fines herbes (tarragon, chervil, and flat-leaf parsley)
3 tablespoons sherry vinegar
1/2 cup olive oil
Juice of 1 lemon
3 tablespoons minced scallions
1 tablespoon coarse-grain Meaux mustard
Salt and freshly ground pepper
[*Editor's note: I would suggest using a red skinned or Yukon gold potato, and any good coarse-grained mustard if you can't find, or choose not to use, the Meaux brand.]
Boil the potato in its skin, then peel and press through the coarse blade of a food mill or potato ricer into a bowl.
Add all the other ingredients and stir to combine, seasoning with salt and pepper to taste.