Cut onion into four. Place beef on top of onion in a deep roasting tray. Place tray in center of the oven for 20 minutes, then turn it down to 375 and cook for 15 minutes per pound for rare, adding 15 more for medium rare and 30 minutes for well done. Baste the beef every so often, but remember to keep the oven door closed. When cooked, take from the oven and place on a board and cover with aluminum foil. This enables the meat to rest and all of the surface juices to go back inside the beef, making it easier to carve.
For the gravy:
Take out approximately 2 Tablespoons of fat from the roasting tray, then pour the rest into a metal container. Pour in about one cup of red wine. This deglazes the pan and helps remove the residue. Sprinkle a little flour into the tray over a medium heat, then blend into a paste with a whisk. Then slowly add stock or water, whisking constantly. Taste, then adjust seasoning.
For the Yorkshire Pudding:
Turn up oven to 400 degrees. In a small mixing bowl, beat the eggs until frothy. In another small bowl, mix the salt and flour. Stir the beaten eggs into the flour, constantly whisking. Gradually pour in the milk. Pour some of the beef dripping into a medium muffin tin or popover tin. Place in over for approximately four minutes until smoking appears. Carefully remove pan from oven and pour in the batter mixture. Place in oven and bake for 20 minutes or until fluffy and golden brown.