Prepare the mushrooms by using a dry hard brush to remove any dirt, then gently remove the stalk to the level of the mushroom inside (i.e. so you form a bowl inside the mushroom). Next fry the bacon until it is nice and golden, then transfer to a paper lined plate to cool. Then wash the flat leaf parsley, remove the leaves and chop finely into a bowl. Continue by shelling the walnuts then using a knife to cut them into small pieces which you then add to the parsley in the bowl. Grind a few twists of salt and pepper then grate the cheddar and add to the mix before stirring thoroughly with the bacon bits. Finally take a spoon and add the mixture to the mushrooms and press down so that the mix is firmly inside. I cooked mine for 10 minutes on indirect heat on a BBQ, but you can also place them under the grill for around 10 to 15 minutes for the same effect.