Heat a generous drizzle of olive oil in a large frying pan over medium-high. Add mushrooms and thyme, season with salt and pepper, and cook until mushrooms are golden brown and tender.
Set aside in a large bowl.Increase heat to high and add a fresh drizzle of olive oil to the same frying pan. Add the steak in a single layer, season well, and let it cook until well browned on each side but still pink in the centre of each slice.
Remove steak from pan, and transfer to a board to rest. Slice into bite size strips that will eat well in pasta.
Reduce heat to medium-high, add a fresh drizzle of olive oil to the pan, and cook onion and garlic until golden and softened.
Add stock and cream, bring to a simmer, and cook until reduced by half. Whisk in sour cream, Worcestershire sauce, and mustard; season with salt and pepper.
Simmer until thick and creamy. Return mushrooms and beef to the pan and stir until just heated through.
Meanwhile, cook pappardelle according to package instructions, until al dente.
Keep one cup of pasta water aside. Drain and transfer to a large bowl. Add butter and a handful of parsley.
Toss until butter is melted and the pasta is well coated and shiny. Add a dash of pasta water while tossing to loosen the noodles and keep them saucey.
Arrange pasta in bowls and spoon stroganoff over top.Garnish with additional parsley and a crack of black pepper.