If using regular yogurt, place it in a strainer lined with paper towel and set the strainer over a bowl. Let the yogurt drain and thicken for 20 minutes.
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.
Tools You May Need
Tools You May Need
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