Biting into a slice of juicy roast sirloin of beef with a crisp brown crust covering a moist pink center is worth the work. Although, truth be told, there isn't that much work involved. Sirloin is a relatively tender cut of beef. It needs a bit more preparation than, say, a filet of beef. Marinating the roast before roasting keeps it juicy and adds flavor.
Combine the red wine, rosemary, garlic and scallions in a zipper locked bag. Place the roast in the bag and then in the refrigerator for at least eight hours up to 24 hours. Turn it over once or twice so the entire roast is exposed to the marinade. Remove from the fridge two hours before roasting. The roast will cook more evenly if it isn't cold in the center.
Drizzle 2 tbsp. of olive oil in a metal baking pan. The bottom and sides should be coated with oil. Place the vegetables on the bottom of pan. Put the rack on top of the vegetables. If you don't have a rack, use crumbled up aluminum foil to keep the bottom of the roast from touching the vegetables.
Preheat the oven to 550 degrees.
Remove the sirloin roast from the marinade. Save the marinade. Put the roast on the rack. Put the pan in the oven and immediately turn the heat down to 350 degrees. The high heat sears the outside of the roast trapping all the juices inside.
Roast for 18 minutes per lb. for medium rare. Roast five minutes longer per lb. for medium and about seven minutes longer per lb. for well done.
Remove the pan from the oven. Put the roast on a plate and loosely cover with aluminum foil. Add the marinade into the pan. Scrape up all the lovely browned bits and the vegetables. Put them in a blender with 1 cup of red wine and 1 cup of good beef broth. Add 1 tbsp. flour for every cup of the mixture. In other words, if you have 3 cups of liquid, add 3 tbsp. of flour. Blend well. Place in a saucepan and bring to a boil over medium heat. Let the mixture bubble for four minutes. Remove and serve as a sauce for the meat.
Take out the roast a few minutes before the cooking time is up. It will continue to roast until it cools a bit.