2 tablespoons white-wine vinegar
1 teaspoon sugar
1/4 teaspoon salt
2 tablespoons extra-virgin olive oil
1/2 lb pickled herring, drained
1 (1-lb) jar sliced pickled beets, drained
1 (6-oz) bunch watercress, coarse stems discarded (4 cups sprigs)
2 medium Belgian endives, cut crosswise into 1/2-inch-thick slices and cores discarded

Whisk together vinegar, sugar, and salt until sugar is dissolved, then add oil in a slow stream, whisking.
Arrange herring and beets on a platter and drizzle with half of vinaigrette.

Toss watercress and endives with remaining vinaigrette, then mound over herring and beets.