The roast can be stuffed and tied the day before you cook it, to let the flavor of the herb mixture permeate the meat.
Heat oven to 450 degrees. Make a vertical cut in side of roast about three quarters of the way through. Open out meat, then repeat cuts at two thickest parts so that you have a thin, flat piece of meat. Season well with salt and pepper.
Combine sage, garlic, and oil. Spread mixture over entire surface of the meat. Roll roast up and tie tightly with string. Rub with a little olive oil and season with salt and pepper.
Place roast, fat side up, in a roasting pan and cook for 15 to 20 minutes, until brown. Reduce heat to 300 degrees and continue cooking for about 1 hour more, or until internal temperature reaches 140 degrees. Baste occasionally with pan juices.
Remove roast from oven and let cool to room temperature. Toss arugula with oil and vinegar and serve with thinly sliced roast.