In a large bowl, mix the cucumbers and salt and set aside at room temperature for 1 hour. In a colander in the sink, drain and rinse the cucumbers thoroughly under cold running water, then drain it well, pat dry, Transfer the cucumbers to a large bowl and mix with the sour cream, dill, 1 teaspoon vinegar, cayenne, and season with pepper, to taste. Meanwhile, toss the onion with the remaining 2 teaspoons of the vinegar in a microwave-safe bowl, and microwave on HIGH for 1 minute. Set aside to cool.
Arrange the fish, cucumber salad, onions, watercress, and bowls of horseradish on a large serving platter. If desired, serve with the multi-grain bread.
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