2) For the fillet of beef, preheat the oven to 240C/Gas mark 9. Melt 2 tbsp of unsalted butter. Place the beef on a baking sheet and pat the outside dry with a paper towel. Using a pastry brush, coat the beef with the melted butter. Sprinkle evenly with salt and pepper.
3) Roast the meat for 25 minutes at 250C/Gas mark 9 for medium rare. Remove from the oven, cover tightly with aluminium foil, and allow to rest at room temperature for 20 minutes before slicing.
To make the sandwiches:
1) Cut the bread into 8 slices, each slice 1cm thick. Spread 4 of the slices thickly with the dressing. Top with slices of beef and rocket and sprinkle with salt and pepper.
2) Spread the rest of the slices of bread very lightly with butter and place, butter-side down, on top of the beef.