Put the tuna with its oil, the beans, onion, capers, watercress, olive oil, vinegar, salt, and pepper in a large bowl. Toss gently to combine.
Variations: · Use arugula instead of the watercress.: · Try canned sardines instead of the tuna.
Wine Recommendation: 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.
Notes: 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.