- 1 tablespoon white-wine vinegar, or to taste
- 1 tablespoon Dijon-style mustard, or to taste
- 1/4 cup olive oil
- 1 tablespoon water
- a 1/2-pound russet (baking) potato
- 1 tablespoon unsalted butter
- 1 small bunch of watercress, rinsed, spun dry, and coarse stems discarded (about 3 cups)
- 1/2 red onion, sliced thin, separated into rings, and soaked in ice water to cover for 30 minutes
- a 3 3/4-ounce can brisling sardines packed in oil, drained well
- In a small bowl whisk together the vinegar, the mustard, and salt and pepper to taste, add the oil in a stream, whisking, and whisk in the water. Peel the potato, grate it coarse, and squeeze it between several thicknesses of paper towels to remove any excess moisture.
- In a non-stick skillet measuring 6 inches across the bottom heat 1/2 tablespoon of the butter over moderately high heat until the foam subsides, add half the potato, tamping it down with a spatula, and cook the potato cake, shaking the skillet frequently to keep the potato from sticking, for 6 minutes, or until the underside is golden and crisp. Turn the potato cake, cook it for 6 minutes more, or until the underside is golden and crisp, and transfer it to a plate. Make a second potato cake in the same manner with the remaining butter and potato and transfer it to another plate. In a bowl toss the watercress with 3 tablespoons of the dressing and spoon the remaining dressing around the potato cakes. Arrange the onion, drained well, the watercress, and the sardines decoratively on the potato cakes.