1. Trim any excess fat from the steak and score the remaining fat at the edges at 1-inch intervals. Place the steak in a large shallow glass dish. Combine the oil, red wine and shallots or onion in a 1-cup measure; pour the marinade over the steak. Allow the steak to marinate at room temperature for at least 2 hours. Remove the steak from the marinade.
2. Prepare the grill so the coals are medium-hot.
3. Grill the steak 5 inches from the heat for 10 minutes on each side for rare, 15 minutes on each side for medium and 20 minutes on each side for well done.
4. Place the steak on a carving board. Spread the steak with a pat of butter or margarine and season with the salt and pepper. Allow the steak to "rest" for 10 minutes. To carve the steak, remove the T-bone and slice across the steak so that each slice will have the back tenderloin and a piece of the loin strip.