Rinse and dry the watercress sprigs and stems and set sprigs aside.
Combine the chopped stems, water, and garlic in a small saucepan. 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 food processor/blender, combine the mayonnaise, sour cream, vinegar, capers and salt and black pepper to taste. Puree until smooth.
Serve tuna mixture on reserved watercress sprigs. Drizzle with dressing.