Mix the egg and flour together, bring to a ball and wrap in clingfilm. Place in the fridge for 15 minutes. (while resting make the pasta sauce)
Place a pan of salted water on the hob, so you can cook the pasta when it is ready.
Dice the onion and and mince the garlic.
Place a pan on a medium heat and sweat down the garlic and onion until they start to soften.
Then add in the mushroom and cook for another 5 minutes until they soften.
Add in the double cream and grated parmesan and stir until all the cheese has melted and the sauce is bubbling. Then turn the heat down to low.
Bring the pasta out the fridge and roll out with a sprinkle of extra flour with a pasta machine or rolling pin.
Once rolled out to a thickness of 6mm, slice into pasta strands like tagliatelle and place in the boiling salted water. Cook for 3-4 minutes before draining but leave some pasta water in a cup as it will be used to loosen the sauce.
Add the cooked pasta to the pan and stir into the sauce, add a little bit of pasta water to get your desired consistency.
Taste and season with salt and pepper. Serve and sprinkle over fresh parsley.