Drain canned fish or seafood in a colander placed in the sink. Allow a few minutes for oil or water to drain.
In a mixing bowl gently break canned fish or seafood into chunks, using a fork. Stir in apple, celery, walnuts, and raisins; set aside.
For dressing, in a small mixing bowl combine sour cream, mayonnaise or salad dressing, and lemon juice. Add to tuna mixture, lightly tossing. Toss chunks of fish gently to avoid breaking them into pieces.
To serve, line 4 salad plates with lettuce, then sprinkle cheese over lettuce. Spoon tuna mixture atop cheese. Makes 4 servings.