Season the steaks with salt and pepper, fry them in heated oil, then keep warm.
Fry the mushrooms and the onions in the remaining frying fat for about 5 minutes. Pour in the crème fraîche and stock, bring to the boil. Add the oregano, simmer for about 5 minutes.
Cook the tagliatelle in salted water until al dente, then drain.
Season the sauce to taste and serve with pasta and steak strips.