Since there are so few ingredients in this simple salad, you'll want to use really good canned beans; lower-quality beans are mushy. Among the many fine brands, we find Goya to be consistently reliable.
If your tuna has less than one-and-a-half tablespoons of oil per can, add a little extra olive oil to make up the difference. We're counting on that much oil as part of the dressing.
•Use arugula instead of the watercress.
•Try canned sardines instead of the tuna.
Vernaccia di San Gimignano, from the vineyards surrounding the oft-pictured hilltop town of San Gimignano, is a refreshing and racy wine. Its subtle almond flavors will blend well with the earthiness of the beans.