Can be prepared in 45 minutes or less.

　　

Ingredients

Serves 2

　　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

　　

Step 1

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.

　　

Step 2

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.