Mix the mustard, horseradish and double cream (if using) in a bowl until well combined. Just before serving, stir it into the beef casserole. Serve with mashed potatoes and steamed vegetables.
For the casserole: Preheat the oven to 375°F ... Refrigerate while you prepare the potatoes so the flavors have time to meld. Place the potatoes into a large pot and cover them with cold water ...