In a 12- by 17-inch roasting pan (at least 2 in. deep), mix potatoes and shallots (unpeeled) with olive oil to coat. Push vegetables to edge of pan and set a V-shaped rack (or flat rack about 10 in. square) in the center; mound vegetables if necessary.
Rinse beef and pat dry; trim layer of fat to 1/4 inch thick.
In a small bowl, mix 2 tablespoons pepper, lemon peel, thyme, 1/2 teaspoon salt, and garlic. Rub mixture all over beef. Set roast, fat side up, on rack in pan.
Roast in a 325° regular or convection oven until a thermometer inserted in center of thickest part of meat registers 135° for medium-rare, about 2 1/4 hours. For medium-well, roast meat until it reaches 145°, about 2 1/2 hours.
Transfer roast to a board or platter. With a slotted spoon, lift vegetables from pan and mound in a bowl or arrange alongside beef. Keeping meat and vegetables warm, let rest 5 to 10 minutes.
Meanwhile, tilt roasting pan to skim fat off drippings and discard. If using tequila, add to pan and set over medium heat. When tequila is warm, remove from heat (and keep away from any vent, fan, or flammables) and ignite with a match. When flames subside, add broth to pan and bring to a boil over high heat, scraping browned bits free. Pour into a bowl.
Sprinkle vegetables with parsley. Carve roast. Serve with pan drippings to spoon over meat and vegetables. Add salt and pepper to taste.