Firstly, thinly slice the red onion and cook in a saucepan over a medium heat with a little oil until softened. Then take off the heat and mix through the Worcestershire sauce. Then set aside to cool.
Preheat the oven to 220C/200C Fan/Gas Mark 7 and line two baking trays with baking paper. Then, in a large bowl, rub the flour, salt and butter together until it resembles breadcrumbs.
Then add the eggs and baking powder and mix to combine. Then add the mustard, Guinness and half the milk, mixing again until fully incorporated. Then add the remaining milk, a little at a time, until it comes together a forms a very soft and wet dough. You may not need to use all of the milk. Finally, mix in the grated cheese and chopped parsley.
Generously dust a clean work surface with flour and tip the dough onto it, sprinkling a little more flour on top of the dough. Flatten the dough out a little with your hands, then using a rolling pin, roll the dough 2/3 times till approx. 1/2 inch thick. Finally, 'relax' the dough by slightly lifting the edges and letting it drop back onto the work surface.
Using an 8cm cookie cutter, cut out rounds by pressing firmly down (without twisting!) and place onto the prepared baking trays, leaving room to spread. Once you've cut as many rounds as you can, re-roll the remaining dough and repeat the cutting process.
In a small bowl, whisk the egg for glazing. Then using a pastry brush, brush the top of the scones with the egg, ensuring it doesn't drop down the sides. Once all have been glazed, repeat the process.
Then place in the oven for 13 minutes until the scones are risen, golden brown and almost fully cooked. (Make sure to swap the trays halfway through the cooking time, if you don't have a fan oven.) Then remove from the oven, and top each scone with some of the cooked onion mixture, and place back in the oven for 2 minutes.