Coarsely chop the watercress stems until you have 1/2 cup. Rinse and dry the remaining watercress sprigs and set aside.
In a small saucepan, combine the chopped stems, water, and garlic. Bring to a boil over medium-high heat. Reduce the heat to low. Cover and simmer until the watercress stems are bright green, about 1 to 2 minutes. Drain and place in a large bowl. Add the tuna, beans, onion, and roasted pepper to the bowl and toss to blend.
In a blender or food processor, combine the mayonnaise, sour cream, vinegar, capers, and salt and black pepper to taste. Puree until smooth.
Serve the tuna mixture on the reserved watercress springs. Drizzle with the dressing.