Sear the roast and start the au jus
The roast will be done when it hits 125 degrees in the center. Before it gets to this point, you want to sear the outside. This will give it a nice crust while keeping the roast nice and juicy on the inside. The goal is to get a nice even sear on all sides without burning the roast.
Take the roast off the grill and place onto a baking sheet and cover with foil.
Turn up the grill to 450 degrees.
While the grill heats up, you can cook the au jus.
Take the roasting pan you used and place it on two burners on your stove set to high heat. Add the stick of butter and mix it with the drippings in the pan. Use a wooden spoon to loosen up the drippings from the roasting pan.
Once the drippings are loosened and mixed with the butter, remove the pan from heat and add them to a medium sauce pan, along with the beef stock, red wine, and rosemary. Whisk together and cook for about 20 minutes on medium heat, stirring occasionally so it doesn’t stick or burn.
Once the au jus is finished, place it in a bowl or gravy boat for serving.
Once the grill reaches temperature, place the roast directly on the grill, fat side down.
Cook the roast fat side down for 5 to 10 minutes. Watch out for flareups! If you have a flareup, remove the roast from the grill, let the flareup die down, and then go to the next step.
Turn the roast onto one of its sides and cook for another 5 to 10 minutes.
Flip the roast onto its other side and cook for another 5 to 10 minutes.
The roast is done (medium-rare) when it reaches 125 in the center.