Ingredients
1.5-1.8kg Chicken 
15g butter, melted 
1 small onion 
1 lemon, cut in half 
Fresh herbs of your choice (optional but parsley, tarragon, rosemary, thyme etc would all work, as would fresh garlic) 
Salt and pepper 

Equipment 
Ideally a meat thermometer

Method
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.  For a chicken you should allow 20 minutes per 450 plus 25 minutes.  This means that for your basic 1.5kg chicken at a fan oven temp of 180c, your chicken should cook for 1 hour 25 minutes, a 1.8kg chicken should take 1 hour 45 minutes. 

Heat the oven to 180c fan. If your chicken has string around it then carefully cut this off as I find it cooks better this way. 

Take the lemon and with a fork poke small holes all over, place into the cavity of the chicken along with the onion and any fresh herbs you plan to use.  

Melt the butter and with ideally a pastry brush (although hands work well too) put butter all over the chicken.  Season generously with salt and pepper and place into the hot oven. 

After 40 minutes, baste the chicken with the pan juices and do this again every 20 minutes.  

Cooking a chicken albeit easy is a tricky in so far as you really don’t want to overcook it because it will go very dry but you do need it thoroughly.  You will often see recipes say 'until the juices run clear' but that is quite difficult to gage I find with a really hot chicken! The most accurate way to tell if your chicken is done is to use a meat thermometer near to the end of the cooking time.  You place the probe into the thickest part of the thigh and your cooked chicken will read 75c.  You can pick a basic meat thermometer, like the Kitchen Craft one, up on line for £5.  Alternatively, if you pull your leg it should start to break away fairly easily. 

When you are happy with how your chicken is cooked, remove it from the pan and loosely cover with foil, resting for 15 minutes.