Slice the meat thinly and season with salt and pepper.
Chop the onion and slice the white mushroom.
Melt the butter in a pan over medium heat and stir-fry the thyme and onion for 2 minutes.
Put meat and white mushroom and cook for 3 minutes.
Add flour and stir-fry for 1 minute. Then add the beef stock little by little and mix until thickened
Add dijon mustard and sour cream and bring it to boil. Reduce the heat to low and simmer for about 5 minutes.
Boil pasta in salted water (boil time as described on the package).
Place pasta and finished stroganoff on the plate.