Using a rolling pin, roll each slice of bread lightly to flatten slightly. Butter slices of white bread on 1 side and slices of whole-wheat bread on both sides.
In a small bowl, mix cheese and chopped watercress until soft and well combined. Spread cheese mixture on 1 slice of buttered white bread. Top with slices of tomato. Season with salt and pepper. Cover with buttered whole-wheat slice of bread. Spread with mayonnaise. Sprinkle liberally with cress.
Spread remaining slice of buttered white bread with salmon pâté. Place, pâté-side down, over cress. Press together firmly. Cut in half crosswise, then cut each half in 6 thin fingers. Garnish with radish roses and sprigs of watercress.