Microwave the potatoes on high until soft, about 20 minutes. Using a fork, mash the potatoes in a bowl with 2 tablespoons butter. Add the milk and mash; season with salt and pepper. Preheat the broiler.
Meanwhile, melt the remaining 2 tablespoons butter in a skillet over high heat. Add the onion and cook until soft, about 4 minutes. Stir in the flour and cook until slightly brown. Gradually whisk in the chicken broth until smooth. Whisk in the Worcestershire sauce and simmer until thick, about 3 minutes.
Toast the bread under the broiler, about 1 minute. Spread the mustard and horseradish on toasts; place each on a plate. Pour some gravy over the toast; top with the roast beef, more gravy, watercress and parsley. Season with salt and pepper. Serve the sandwiches with the potatoes.
Tools You May Need
Photograph by Antonis Achiellos
Courtesy of Food Network Magazine
Tools You May Need
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