Let the steaks stand at room temperature for 30 minutes before grilling and while you prepare the sauce.
Heat the oil in a saucepan over medium heat. Add the shallot and sauté until translucent, 2 to 3 minutes. Add the wine, stock, and rosemary sprig. Bring to a boil and boil the liquid until reduced by about half, about 15 minutes. Whisk in the vinegar and sugar and remove from the heat. Strain the sauce through a fine-mesh sieve, pressing down on the solids to extract the juices; discard the solids. Whisk in the butter, 1 tablespoon at a time, until smooth. Season with salt and black pepper, and taste for seasoning.
Prepare the grill for indirect cooking over low heat (250° to 300°F).
Brush the oil onto both sides of the steaks and then season evenly with salt and pepper.
Grill the steaks over indirect low heat, with the lid closed, until an instant-read thermometer inserted into the center of a steak, without touching the bone, registers (100°F), 15 to 18 minutes. Transfer the steaks to a platter.
Raise the heat of the grill for direct cooking over high heat (475° to 525°F). Return the steaks to the sear grate and cook over direct high heat, with the lid closed, until an instant-read thermometer inserted into the center of a steak, without touching the bone, registers (125°F) for medium rare, 5 to 7 minutes, turning once.
Remove the steaks from the grill and let rest for about 5 minutes. Cut the meat away from the bone. Next, cut the meat across the grain into thick slices. Serve the steaks warm with the sauce.
