Neatly dicing the cucumber:

Peel the cucumber and halve it lengthwise; scoop out the seeds with a teaspoon. Cut the halves into quarters and the quarters into crosswise pieces 2 inches long. Finally cut the quarters into matchstick julienne, stack the julienne, and cut into dice.

The sauce:

Toss the cucumber in a bowl with the salt, sugar, and vinegar. Let stand 5 minutes or so, then fold in the sour cream. Season carefully to taste, and fold in the dill or watercress.

The fish: Poach the fish as directed below.

To serve hot:

Remove and drain the fillets; arrange on hot plates or a platter. Either decorate with sprigs of fresh dill or parsley and pass the sauce separately, or spoon the cucumber sauce decoratively over the fish.

To serve cold:

Let the fish remain in its poaching liquid at least 20 minutes, to pick up flavor. Remove, drain, and chill. Serve on a bed of greens, and decorate with the sauce. Serves 6.

Ahead-of-time note: May be completed hours in advance; cover and refrigerate.

cold.