Start by making a roux; melt the butter in a medium saucepan and once gently foaming whisk in the flour. Cook for a couple of minutes, stirring constantly, until the flour is lightly browned
Meanwhile bring the beer to the boil. Gradually pour the beer into the roux over a medium heat, whisking continuously until you have a thick, smooth sauce. Remove the pan from the heat and stir through all the cheese as it melts. Add the mustard, Worcestershire sauce and truffle oil. Taste and season with pepper. You may want to add more truffle oil depending on your taste
Preheat the grill
Lightly toast the crumpets in a toaster. Divide the cheesy rarebit mixture between the crumpets and place under the grill for 3-5 minutes or until golden brown and bubbling
Serve the crumpets hot with a sprinkling of chopped parsley.