1 bunch radishes, very thinly sliced, divided
1/2 English cucumber, peeled and thinly sliced
In a food processor fitted with a metal blade, combine goat cheese and watercress (and truffle oil, optional) and pulse until combined. Season with salt and pepper to taste. Place bread on a work surface. Spread each slice with goat cheese mixture, dividing equally. Set aside 24 radish slices for garnish. Top 6 bread slices with cucumber slices, then remaining radish slices. Sprinkle with pepper. Top with remaining bread slices, cheese side down. Trim sandwich crusts. Cut each sandwich into 4 squares. Transfer to a platter, top each sandwich with a radish slice and serve.