1. Preheat oven to 400 degrees. Using a serrated knife, slice off top quarter of garlic, exposing as many cloves as possible. Place garlic, cut-side up, on a piece of parchment-lined foil. Drizzle with 2 tablespoons oil, wrap tightly, and roast until cloves are tender, about 40 minutes. Let stand until cool enough to handle.

2. Squeeze garlic to push out cloves; transfer to the bowl of a food processor. Add parsley, sage, vinegar, and remaining 2 tablespoons oil. Process until smooth. Season with salt and pepper.

3. Arrange roast on a large cutting board, fat-side down, with short end toward you. Holding a long, sharp knife parallel to (and about 1 inch above) cutting board, make a shallow cut along length of roast, stopping about 1 inch from edge. Open roast to the side, and continue slicing until about 1 inch from next edge. Open to same side again, and continue slicing until roast is completely flat. Season meat all over with salt and pepper. Spread all but 2 tablespoons of garlic-herb mixture over meat. Roll meat up, starting from left side. (Fat cap should end up on top of roast.) Tie at 2-inch intervals with kitchen twine. Rub outside with remaining garlic-herb mixture. Let stand at room temperature 1 hour (or refrigerate, covered with plastic, overnight; bring to room temperature before cooking).

4. Preheat oven to 400 degrees. Place meat on a rack in a roasting pan and roast, rotating pan once, until meat is golden brown, 40 minutes. Reduce heat to 300 degrees and continue roasting until a thermometer inserted in thickest part reads 125 degrees, 50 to 60 minutes. Let rest at least 30 minutes before slicing.