how to bake sirloin roast
1 Cook for 15 to 20 minutes (depends on size of roast) at 450 degrees F. 2  Then, without opening the oven door, turn the heat down to 325 degrees F. 3  Cook until meat thermometer inserted reaches about 140 degrees. 4  Generally this should take about 2Â½ to 3 hours, but it varies on oven and roast size.