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.