Shrimp and Herb Canapés

Name: Shrimp and Herb Canapés
Ingredients: 2 cups packed watercress leaves; 1 cup packed parsley leaves; 1 cup packed tarragon leaves; 1 lb. (4 sticks) unsalted butter, softened; 4 tbsp. fresh lemon juice; Kosher salt and freshly ground black pepper, to taste; 8 oz. unpeeled medium shrimp; 1 loaf Pullman bread, cut lengthwise into six ¼″-thick slices, crusts removed; 1 tbsp. capers, rinsed, for garnish; 4 cooked, peeled medium shrimp, thinly sliced lengthwise, for garnish; 4 radishes, thinly sliced, for garnish; 3 chives, cut into 2″ lengths, for garnish