Heat the oil or butter in a large frying pan over a medium-high heat.
Add the minced beef and cook, breaking it up with a spoon, until browned and no longer pink, about 5–7 minutes. Drain off any excess fat.
Add the chopped onion and sliced brown mushrooms. Cook, stirring occasionally, until the onion softens and the mushrooms release their juices and start to brown, about 5–7 minutes.
Stir in the garlic and cook for about 1 minute until fragrant.
Sprinkle over the plain flour and cook for 1 minute, stirring.
Gradually pour in the beef stock, stirring constantly to avoid lumps.
Stir in the Worcestershire sauce, Dijon mustard, salt and black pepper. Bring to a gentle simmer, then reduce the heat to low and cook, stirring occasionally, until the sauce thickens, about 5–7 minutes.
Meanwhile, cook the egg noodles in a large pan of salted boiling water according to the packet instructions until al dente. Drain well.
Remove the pan from the heat and stir in the sour cream until the sauce is smooth.
Taste and adjust the seasoning if necessary.
Serve the stroganoff sauce spooned over the egg noodles and sprinkle over the chopped parsley.