Ingredients

• 1 1/2 pounds red-skinned potatoes, pierced with a fork
• 4 tablespoons unsalted butter
• 3/4 to 1 cup milk
• Kosher salt and freshly ground pepper
• 1/2 medium onion, sliced
• 3 tablespoons all-purpose flour
• 2 1/2 cups low-sodium chicken broth
• 2 teaspoons Worcestershire sauce
• 4 thick slices sourdough bread
• 3 tablespoons dijon mustard
• 1 tablespoon prepared horseradish
• 3/4 pound sliced roast beef
• 1/2 bunch watercress, woody stems removed
• 1/4 cup chopped fresh parsley

Instructions

1. 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.
2. 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.
3. 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.
4. Photograph by Antonis Achiellos