Clean and slice the mushrooms then trim and slice the beef.
Add the olive oil and butter to a large pot over medium heat. When the butter has melted, add the mushrooms and onions. Season with salt and pepper and sauté several minutes until the onions have softened and the mushrooms are cooked then transfer everything to a bowl.
Add the beef in a single layer to the same pot working in batches if needed so you don’t overcrowd the beef. Increase the heat to medium-high, season generously with salt and pepper then sear on both sides. The beef will still be pink inside when you’re finished searing.
Remove the beef then deglaze the pot with the red wine, cooking 2 minutes to let the wine reduce.
Add the Dijon mustard and Worcestershire sauce then simmer 5 minutes over medium to medium low heat.
Add 2 cups of the beef stock, scraping any brown/flavor bits off the bottom of the pot then return the beef to the pot.
Lay the thyme sprigs on the top then cover and simmer over low heat 45 minutes. Remove the thyme sprigs and return the mushrooms and onions to the pot. Cover and cook 15 more minutes or until the beef is fork-tender.
In a small bowl whisk the remaining 1/2 cup of beef stock and the flour together then fold into the sauce, simmering over low heat as it thickens.
Temper the sour cream (see body of post for how to temper) then slowly add it along with the chopped parsley to the sauce stirring to completely blend. Optional: garnish with extra parsley.
Cook the egg noodles according to package directions. Serve the stroganoff over the noodles.