Rinse tomatoes (peel if desired) and chill thoroughly in refrigerator.
Drain 1 can of sardines, remove tails, and cut the sardines into pieces; reserve the other can of sardines for completing the salad.
Lightly toss the sardines with the eggs and capers.
Combine the sour cream, mayonnaise, caper liquid, dry mustard, and salt.
Add to egg-sardine mixture and toss lightly to mix well.
When both the mixture and tomatoes are thoroughly chilled, cut a slice from the top of each tomato.
Using a spoon, remove seeds.
Invert shells to drain.
Sprinkle insides with seasoned salt before stuffing.
Fill center of tomatoes with the egg-sardine mixture.
Choose greenest lettuce leaves and use to line salad plates.
Set tomatoes on lettuce.
Place one sardine across each stuffed tomato and garnish with sprigs of watercress.