Roast Beef Sandwich with Watercress and Horseradish Mayonnaise

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1/4 cup nonfat Greek style yogurt or 1/3 cup regular, plain nonfat yogurt

2 tablespoons mayonnaise

1/4 cup prepared white horseradish, squeezed of excess juice (about 3 tablespoons)

1/4 teaspoon salt

1/8 teaspoon pepper

1 bunch watercress, thick stems removed

8 slices whole-grain pumpernickel bread

12 ounces lean roast beef, thinly sliced

2 medium tomatoes sliced

1/2 small red onion, thinly sliced