Poke the meat with a knife or thick skewer to make small holes all over. Set aside.
In a small mixing bowl, combine the bouillon cubes, the garlic, mustard, tomato paste, 1/8 teaspoon pepper, and the Worcestershire. Pour in the boiling water to dissolve the bouillon. Using the back of a wooden spoon, crush the bouillon cubes. Keep crushing and stirring until you've made a nice thick paste.
Rub the paste all over the meat, rotating it to coat all sides well. Transfer meat to a roasting pan, cover loosely with foil, and marinate at least 30 minutes and up to 1 hour.
While meat is marinating, set a rack in middle of oven and preheat oven to 425 degrees. Pour enough water into the roasting pan to come to a depth of 1/4 to 1/2 inch. Place pan in oven and roast, covered, 15 minutes.
Remove foil and rotate pan. Continue cooking and check after 10 minutes that there is still liquid in the pan; add ¼ cup hot water if needed.
Roast an additional 5 minutes for rare, an additional 15 minutes for medium-rare, and an additional 20 minutes for medium, or until center of roast registers the desired temperature on an instant-read thermometer. Rare: 125 degrees (total cooking time about 30 minutes); medium-rare: 130 degrees (about 40 minutes); medium: 140 degrees (about 45 minutes).
Transfer roast to cutting board, cover with foil to keep warm, and allow to rest.
Prepare gravy: Place roasting pan with drippings on a burner over medium heat. Scrape up the brown bits with a wooden spoon. Add butter and stir.
Whisk in the wine, sherry, and beef broth. Add a cornstarch-water mixture, whisking until thickened, 2 to 3 minutes.
Slice the roast. Serve on a platter over roasted green beans (if using). Serve gravy separately.