Melt butter in a large skillet over medium high heat and once completely melted add mushrooms and onion to the pot and cook, stirring occasionally, until mushrooms are tender, 5-7 minutes.
Add the flour and mix until dissolved then add the stock to the pot, cook on medium high heat for 7-10 minutes.
Add the mustard, seasonings, and cook through for 3-5 minutes. Add flour or stock if needed to adjust the thickness of the sauce.
Turn the heat off, add the cooked pasta and sour cream, stir thoroughly and adjust seasoning.