In a small bowl add 3 tbsp mustard and the sugar and stir to combine. Spread over the top of the beef joint before seasoning generously with salt and pepper. Heat oil in a large frying pan over high heat; sear on all sides, until sealed and well browned. Then transfer to a snug-fitting roasting tin and tent with foil.
Place in a pre-heated oven (200c / 180c fan) and cook for 25 minutes for rare meat or 30 - 35 minutes for medium/ medium-well according to preference.
Whilst the beef is in the oven, toast the bread on both sides on a grill pan and allow to cool. When ready, remove the beef from the oven, place it on a carving board and allow to cool a little before carving into fine slices.
Assemble the sandwiches by spreading the toast with a layer of the mustard, mayonnaise and slices of roast beef, top with a scattering of red onion and the watercress.