1. Preheat oven to 450 degrees F. Season beef all over with 1/2 tsp. of the salt, 1 tsp. of the pepper, and 2 Tbsp. of the fresh thyme (or 2 tsp. dried). Using a small, sharp knife, make small slits in the fat along the top of the beef and gently insert the garlic slices. Place the beef on a rack in a large roasting pan fat side up. Roast the beef for 20 minutes.
2. Place onions and potatoes in a bowl and toss with remaining 1/2 tsp. salt, 1/2 tsp. pepper, and 1 Tbsp. fresh thyme (or 1 tsp. dried).
3. Reduce oven temperature to 325 degrees F and roast the meat for another 30 minutes, then add the potatoes and onions to the pan, surrounding the beef. Stir the vegetables well to coat with juices collected on the bottom of the roasting pan. Continue roasting until the internal temperature reaches 140 degrees F for medium-rare meat, about 1 hour and 30 minutes more. Baste the beef and the vegetables once or twice so they brown evenly. (Check the meat's internal temperature with an instant-read thermometer. The beef will continue to cook once it's removed from the oven, so be careful not to overcook it.)
4. When meat reaches the desired temperature, remove from oven and place on a carving board; cover loosely with foil for 10 to 15 minutes before carving. Keep the oven hot for the Yorkshire pudding while the beef rests. Place a 12-cup muffin pan in the oven.
5. Prepare the Yorkshire pudding. In a large bowl whisk eggs vigorously or beat well with an electric mixer. Add milk and whisk again, making sure the mixture is airy and light. Gradually add flour, salt, and pepper and beat until the mixture is smooth. (You can make the batter a few hours ahead of time; cover and refrigerate.)
6. Remove pre-heated muffin pan from the oven. Raise oven temperature to 450 degrees F. Add 1 tsp. of the beef drippings from the roasting pan to each muffin cup, or use the same amount of melted butter instead. Return the pan to the oven to heat up for another 2 to 3 minutes.
7. Remove pan from oven and carefully divide the batter among the muffin cups on top of the hot fat and place pan on the middle shelf of the oven. Bake for 10 minutes. Without opening oven, reduce the heat to 350degreesF and bake another 10 to 15 minutes. The pudding should rise and begin to turn golden brown.
8. While the pudding is baking, place meat on a platter and surround with potatoes, onions, and thyme, if using. Pour any remaining pan juices on top. When Yorkshire pudding is ready, remove from pan and place in a shallow serving bowl. Serve immediately.
Source: Hannaford fresh Magazine, November - December 2010