1 Put the sirloin in the oven and roast it for about 15 minutes for a medium-rare roast, 20 minutes for a medium roast, 25 minutes for a medium-well-done roast or 30 minutes for a well-done roast. 2  Turn the beef over about halfway through cooking [source: Rhodes ].oasts are relatively easy dishes that need minimal seasoning for a tasty dinner. Here's a delicious recipe for sirloin roast. Ingredients: 1  2.2 to 3.3 pounds (1 to 1.5 kilograms) sirloin of beef. 2  2 tablespoon vegetable oil or beef fat.