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Combine thickened or Greek-style yogurt, mayonnaise, horseradish and salt and pepper in a small bowl and stir to incorporate.
Place 1/4 cup watercress each on 4 slices of bread and drizzle with 2 tablespoons horseradish mayonnaise. Layer with 3 ounces roast beef per sandwich, tomato slices and onion slices. Top with another slice of bread and wrap in waxed paper, then aluminum foil.