Bring a large pot of salted water to boil.
Heat a large pan to medium and add 3 tablespoons of olive oil and the mushrooms. Cook the mushrooms until they release their water and brown, then season with salt and pepper to taste and remove them to a bowl.
Add the remaining olive oil, 3 tablespoons of butter, and the onions to the pan and saute until soft (about 10 minutes). Add the garlic and cook for another 2 minutes or until very fragrant.
At this time begin cooking the egg noodles to the package instructions and when finished toss with 3 tablespoons of butter. Make sure to reserve at least 2 cups of pasta water.
Add the flour to the pan and cook for 1 minute or until all of the white specks are gone.
Add the white wine and beef stock to the pan and turn the heat to high. With a flat wooden spoon, scrape the bottom of the pan to dislodge all of the brown bits.
Bring to a boil while whisking until smooth for 3 minutes. Lower the heat to a simmer and add the mushrooms, and sliced steak to the pan along with the mustard and Worcestershire sauce. Continue to cook at a low simmer while waiting for the noodles.
Add the heavy cream and sour cream to the pan and continue to simmer for 2 minutes. Taste test the sauce and adjust salt and pepper if required. If the sauce is too thick add a few ounces of the reserved pasta water. When satisfied remove the pan from the heat.
Divide the noodles into bowls and ladle the stroganoff over each one. Garnish with parsley. Enjoy!