Take your beef joint out of the fridge about 30 minutes before you want to cook it, so that it can come up to room temperature.
Preheat your oven to 220 degrees. Place the beef in a medium roasting tin. Drizzle the olive oil over the top of the meat and rub over the skin. Rub the mustard powder over the top of the olive oil to create a nice crust, and then season with salt and pepper.
Place the beef in the centre of the oven and cook for 20 minutes at 220 degrees – set a timer so you know when to turn the heat down.
Now prep your potatoes whilst your beef starts cooking. Peel the potatoes and cut into large chunks. Put them in a pan of boiling water with a pinch of salt and the baking soda, and cook for about 7 minutes.
Once cooked, drain and return to the pan with the seasoning. Put a lid onto the pan, and holding it firmly shut, shake the pan to ‘fluff’ the outside of the potatoes, which will create the crispy bits! Add 2 tsp of gluten free flour and shake again, then set to one side.
After the beef has been cooking for 20 minutes, reduce the temperature to 180 degrees and place a roasting tin with the olive oil into the oven to heat up ready for the potatoes.
Put a timer on and cook the beef for a further 30 minutes, basting the meat at the halfway point.
After 5 minutes remove the potato pan from the oven and carefully tip the potatoes to the roasting tin – take care as the fat may spit. Use some tongs or a spoon and fork to turn the potatoes over so they are coated in the fat. Arrange in a single layer and cook for 15 minutes before turning over and cooking for another 15 minutes.
When the beef is ready, remove from the oven and cover it loosely with a piece of foil to rest for about 20 minutes before carving.
Turn the oven back up to 200 degrees, add the rosemary to the potatoes tin, give them a shake and cook for a further 10 minutes or until they are as crispy as you like them.
While the beef is resting, steam your greens and heat the gravy. When the potatoes are ready, drain off the excess fat on some kitchen towel.
Slice the beef and serve with the roasted potatoes, greens and plenty of gravy.