Pre-heat oven to 140c, heat a fry pan to a hot heat and sear the roast beef in a little oil until golden on all sides. Transfer to a baking dish, smear all over with half the seeded mustard and roast slowly in oven for approximately 20 -30 minutes (using a meat thermometer look for medium rare 52c internal, medium 58c internal, med well 63c internal). Rest on a cooling rack, loosely covered with foil for 20 minutes before serving.
Spread the outsides of the dark rye slices with butter, Slice beef and assemble the sandwiches with a layer of mustard beef, sauerkraut & cheese.
Heat a sandwich press or non-stick fry pan to a medium heat, and fry until golden and the cheese begins to melt. Serve with pickles on the side.