Set the pasta to cook in a pot of salted boiling water according to package instructions.
Meanwhile, heat the oil and butter in a large pan and brown the sliced mushrooms with a pinch of salt and pepper until golden over medium heat. If your mushrooms release too much liquid and start to steam, turn up the heat to get them browned.(Read my tips in the post!)Remove the mushrooms to a separate plate and set aside.
To the same pan add chopped onions and saute over low heat for 7-10 minutes until tender and slightly golden. Add minced garlic and cook for about 30 seconds longer, then hit the pan with a splash of whisky and let it simmer until the alcohol is mostly burned off and the liquid is reduced in half.
Then add 1 tablespoon of flour and mix well until it's combined with the fat. Then pour in the stock and bring to a boil, bring the mushrooms back to the pan and mix to combine. Then add double/heavy cream, Dijon mustard and one bay leaf. Let simmer over low heat for about 10 minutes. Then turn the heat off and stir in sour cream (this prevents sour cream from curdling), season with salt and pepper.
Drain the pasta and add to the mushroom stroganoff, toss to combine until pasta is evenly coated with the sauce. Serve sprinkled with chopped parsley.