Roast beef and creamed horseradish sandwich

Roast beef
Preheat oven to 475F (240°C)
It depends on what cut you have chosen, but I like to use a joint
with a fat layer as it provides basting juices to keep the meat
succulent and moist. Mix the mustard and flour and rub all over the
piece of meat. Season with salt and pepper.
Place the piece of meat on the onions halves in a roasting tin.
During roasting time, the onions will caramelize and impart a fantastic
flavor to the meat. For about 30 minutes cook the meat on the set
temperature. Then turn the heat down to 375F(190°C) and cook the meat to
your liking.

Delia Smith has the following guide:
15minutes per 450g – rare
Add 15 minutes to total cooking time for medium rare
Add 30 minutes for well done.

Baste the meat every now and then to keep it moist.
To see if the meat is cooked to your like, insert a skewer in the
thickest part of the meat. The juices will indicate how well the meat is
cooked.
Once the meat is cooked to your liking, remove it from the heat and
let the meat rest for at least 30 minutes. Cover loosely with tin foil.

Creamed horseradish
Mix all the ingredients in a ceramic bowl about 1 hour before serving.

To assemble
Cut a thick slice of rye bread, butter and
layer with thin slices of roast beef, peppery rocket or watercress,
avocado,capers and finish off with a big dollop of creamed horseradish.