Try to get only the best ingredients for this salad. It's a simple salad made delicious only by the best tuna, and the best tuna for this salad is imported from Italy or Sicily (Spain or Portugal is fine, too) packed in oil. If you can't find it, some grocers or delis carry American brands packed in oil. (I guess what I'm saying is---"packed in water" gives you no flavor whatsoever!) A good substitute for the cannellini beans is great northern.
2 cans Italian Cannellini beans (15 oz.) -- drained and rinsed
1 medium red onion -- chopped
1/4 cup choped fresh flat-leaf parsley
3 tablespoons extra virgin olive oil
3 tablespoons red wine vinegar
Kosher salt and fesh ground pepper
1 can solid tuna fillet packed in oil (6-oz.) -- drained
Combine beans, onion and parsley in salad bowl.
In a small bowl, whisk together the oil and vinegar. Season to taste with salt and pepper, and pour over beans, using only enough to coat the ingredients (don't make it soupy!). Cover and chill completely.
Line individual serving plates or a medium serving bowl with lettuce leaves. Spoon bean mixture into center. Top with chunks of tuna and pass additional salad dressing on the side.