If you are using bottled, prepared horseradish, drain it through a fine sieve, pressing out the excess juices with a wooden spoon or squeeze it dry through a kitchen towel or double thickness of cheesecloth. In a large mixing bowl, combine the horseradish, sour cream, salt, pepper, onions, vinegar and 2 tablespoons of the chopped dill. Break the fish into 5 cm chunks and carefully fold it into the sour-cream dressing with a rubber spatula. Marinate for at least 30 minutes in the refrigerator, then arrange the fish, sauce and all, on a bed of dried, chilled lettuce leaves. Garnish with the sliced eggs and tomato wedges and, just before serving, strew the remaining tablespoon of chopped dill over the salad.