Roast Beef and Yorkshire Pudding with Horseradish Relish

Place the joint on a chopping board, score the skin, pat dry with absorbent kitchen paper and rub the mustard over the surface of the joint.  Season with salt and pepper.
Place the joint on a metal rack in a large roasting tin, add the onion and garlic and open roast for the preferred calculated cooking time, basting occasionally with any rich beefy juices.  If the bottom of the pan seems a little dry, add a little extra beef stock or water.
1 hour before the end of the cooking time prepare the Yorkshire pudding.  In a large bowl sift the flour and salt together.  Make a well in the centre of the bowl and whisk in the eggs, thyme and enough milk to form a smooth, thick batter, about the consistency of double cream.  Cover and leave the batter to rest in the refrigerator for about 30 minutes.
To prepare the roast potatoes, heat a large roasting tin with the oil or dripping in the oven for 5 minutes.  Par-boil the potatoes in a large pan of boiling water for 10 minutes.  Drain and shake the saucepan, to give the potatoes a rough texture.
Remove the roasting tin from the oven and carefully add the potatoes.  Toss gently, return to the oven and cook for 45-50 minutes until golden brown.
When the beef is cooked transfer the beef to a platter and loosely cover with foil and keep warm.  Retain the contents of the roasting tin for the gravy.
To cook the Yorkshire puddings: increase the oven temperature to Gas mark 7, 220°C, 425°F and add the oil or beef dripping evenly in a 12-hole muffin or Yorkshire pudding tin and preheat in the oven for 5 minutes.
Re-whisk the batter, just before using, adding any extra milk to return to the consistency of double cream and pour into a jug. Remove the hot tin from the oven and evenly pour the batter into each hole, return to the oven and cook for 30-35 minutes, or until golden brown and crisp.
Meanwhile, prepare the gravy, spoon off any excess oil from the tin, leaving about 30ml (2tbsp) of any rich, beefy juices.  Place the roasting tin over a medium heat and sprinkle over the flour.  Stir well with a small whisk or spoon, add a little stock and stir again, scraping the base of the pan to release any sediment.
Add the remaining stock, wine and any meat juices from the platter, adjust the seasoning, if required and simmer for 8-10 minutes, stirring occasionally or until reduced to a well-flavoured gravy.  Strain before serving.
To prepare the horseradish relish, mix all the ingredients together, spoon in to a serving bowl and serve as an accompaniment. Tips: For extra flavour add a crumbled vegetable stock cube in the water when boiling the potatoes or add add 1-2 sprigs of fresh rosemary.
For a really crunchy texture dust the potatoes in plain flour or fine polenta (cornmeal) after par-boiling.
For extra rich beefy flavour use the oil from the joint to roast your potatoes.