In a medium bowl, mix together tuna, beans, cheese cubes, onion and grated Parmesan. Prepare dressing and pour 1/4 cup over bean mixture; toss well. Cover and refrigerate 4 hours, stirring occasionally. If desired, serve on lettuce and garnish with tomato.
Lemony Italian Dressing: In a small glass jar, combine all dressing ingredients and shake well. Refrigerate unused dressing. Makes 1 1/4 cups.
Makes 6 to 8 servings.