In the same skillet set over medium heat add 1 tablespoon olive oil and 1 tablespoon of butter. Add the onion and cook, stirring often, for 6-8 minutes, or until golden and starting to caramelize slightly. Add the minced garlic and continue to cook for one minute, stirring continuously.
(optional) Add the dry white wine and continue to cook until most of the liquid has evaporated. In the meantime, scrape any brown bits stuck to the bottom of the skillet.
Prepare the roux. Make a well in the center of the skillet and melt the remaining 3 tablespoons of butter. Whisk in the flour and continue to whisk until the flour and butter are completely incorporated.
Stirring continuously, slowly pour in the chicken stock. Whisk in the Dijon mustard.
Return the mushrooms and chicken thighs (with their juices) back to the skillet and bring to a simmer. Reduce heat to low. Simmer for 2-3 minutes.
Carefully mix the sour cream with the chicken and mushrooms and return to a simmer Simmer for 1-2 minutes and season with additional salt and pepper, to taste. Toss with pre-cooked egg noodles or serve with noodles of your choice on the side. Garnish with fresh chopped parsley.