The wing rib is basically the bit between the sirloin and the forerib. If you can't get your hands on the Bickleigh wing rib from Selfridges Food Hall (because I've nicked it all for the Oyster and Chop House), then a good butcher should be able to supply you with one. A two-bone rib should be enough for 4-6 servings and it doesn't have as much fat surrounding the eye of meat as a fore rib so it's perfect for a cold roast joint.
A two-bone wing rib weighing about 1.5kg
Sea salt and freshly ground black pepper
A little vegetable oil for brushing
For the mustard mayonnaise
4-5tbsp good quality or home-made mayonnaise
2tbsp grain mustard
1tbsp Dijon or English mustard
Pre-heat the oven to 220C/gas mark 7. Cut the wing rib in half through the bones, then season well and brush with oil. Heat a heavy roasting tray on the stove and brown the beef on all sides. Transfer to the oven and cook for about 20 minutes for rare and another 10 minutes for medium rare. Remove from the oven and leave to cool.
Mix all of the ingredients for the mustard mayonnaise and transfer to a serving dish.
Take a wooden carving board and a sharp knife to your picnic and slice the beef when you're ready to serve it.