Core tomatoes; cut a thin slice from top and bottom ends so tomato slices will stand flat. Cut tomatoes in half crosswise to make four thick slices. Place tomato slices on serving platter; set aside.
In a small bowl stir together sour cream, snipped dill or dried dillweed, shallot, horseradish, white wine Worcestershire sauce, salt, and white pepper.
Pile smoked salmon atop tomatoes. Top with a dollop of the sour cream mixture and dill sprigs. Makes 4 appetizer servings.