For the beef: Allow the beef to stand at room temperature before roasting, 30 minutes. Place an oven rack in the center of the oven. Preheat the oven to 425 degrees F. Mix together the thyme, salt, pepper and garlic in a small bowl. Make 1-inch-long, 1/2-inch-deep slits all over the meat using a paring knife. Rub the garlic mixture into the slits and onto the rest of the meat. Place the meat, fat-side up, in a shallow roasting pan. Roast for 45 minutes. Cover the meat with foil and continue to roast until a meat thermometer inserted straight down into the center of the meat registers 145 degrees F for medium-rare, 65 to 75 minutes. Place the roast on a baking sheet and tent with foil. Allow to rest for 20 minutes.
For the sauce: Combine any juices from the rested meat with the broth, marsala, arrowroot and thyme in a medium saucepan. Bring to a boil, stirring constantly. Reduce the heat and simmer until thick, 20 minutes. Whisk in the butter until smooth and season with salt and pepper. Pour the sauce into a serving pitcher. Slice the roast into 1/2-inch-thick slices and arrange on a platter. Serve the sauce alongside.