Ingredients
1-1½kg Sirloin of beef joint 
1 tbsp porcini mushroom powder 
15g butter, melted 
Salt and pepper 

Equipment 
Ideally a meat thermometer 

Unless you buy your meat pre-packaged from a supermarket which gives you an exact cooking time then you will need to calculate how long the cooking process is going to take.  The times for cooking a joint is 10-15 minutes per 450g for medium rare, 15-20 minutes per 450g for medium, 20-25 minutes for medium-well and 25-30 minutes for well done.  This makes it fairly tricky for a new starter so I tend to err on the latter time so 15 minutes per 450g for medium rare, 20 minutes per 450g for medium, 25 minutes for medium-well and 30 minutes for well done but this is also because I know my oven is a little slower.   

Using this calculation this means that for your basic 1kg joint to cook to medium at a fan oven temp of 180c your beef should cook for 50 minutes, a 1½kg joint should take 1 hour 10 minutes. 

Heat the oven to 180c fan. Rub the porcini powder all over the outside of the beef then season with salt and pepper.  Put the roasting pan into the oven.  Heat the oil in a frying pan until it is really hot and then put the beef into the pan, fat side down and colour all over.  This will take 5-10 minutes in total to brown each side. 

Put the beef into the roasting pan, fat side up.  After 25 minutes, turn the beef over. 

The most accurate way to get your beef to the way you like it cooked is to use a meat thermometer where you place the probe into the middle part of the joint near to the end of the cooking time, this allows you to see how it is cooked exactly.  Your beef will read 52c for rare, 57c for medium rare, 60c for medium, 65c for medium rare and 71c for well done. You can pick a basic meat thermometer, like the Kitchen Craft one, up on line for £5. 

Remove the beef from the pan and loosely cover with foil, for 20 minutes.