Slice two red onions into eighths and lay along the bottom of your baking tray. Drizzle lightly with oil.
Salt and pepper both sides of your beef and lay on top of the onions.
Spread the mustard generously over the fatty side of the beef.
Cook at 200C for 20 minutes. Then flip the beef and cook for a further 20 minutes at 160C (Both temperatures for fan ovens).
After 40 minutes pour a glass of decent red wine into the roasting dish and roast for a further 20 minutes at 160.
Take the meat out of the oven and stand for 15-30 minutes.
To make the gravy, add a teaspoon of dijon mustard, 2 tea spoons of gravy granules and two tablespoons of water into a pan on a low-medium heat. Pour in the juices and the red onions from the bottom of the roasting dish and allow to thicken gently. Serve generously on top of the meat.
For beef, it's essential to cook on a high heat for 20 minutes to start off with. If your meat isn't exactly 1kg, in addition to the initial first 20 minutes, roast for 15 (rare) or 20 (medium) minutes per 450g.