Open butterflied roast onto a board, with the fat-cap down, allow to come to room temperature for 20 minutes.
In a medium bowl, combine the stuffing ingredients and stir to combine.
Spread stuffing evenly over beef, leaving a 2cm border around the edges. Roll tightly to enclose, and with the fat-cap on top, secure with butchers string every 3cm. Season with salt and pepper, rub with seeded mustard, drizzle with lemon juice and olive oil.
Transfer roast to a shallow gastronorm. Roast beef at 160◦C dry heat to an internal temperature of 56◦C. Remove from oven, cover loosely with foil, and rest for 10 minutes.
