Heat up the butter in a large, deep-sided skillet over medium heat. Add the onions and mushrooms and season them with salt and pepper. Cook for 5-7 minutes, stirring often, until the liquid has evaporated and the veggies are tender. Add the garlic and cook for 1 minute until fragrant. Transfer the veggies to a bowl and set aside.
Turn the heat up to medium-high and add the oil to the same skillet. Season the steak with salt and pepper. Sear the steak in batches for 1 minute on each side, about 4 minutes total, to get a good sear on the meat. Add more olive oil between batches if needed. (see note)
Add all the steak back to the skillet. Sprinkle the flour over the meat and stir to coat. Cook for 1 minute and then pour in a beef broth. Be sure to scrape up all the browned bits on the bottom of the skillet.
Add the Worcestershire, Dijon mustard, and bay leaves to the skillet. Bring to a boil. Then reduce the heat to medium and simmer for 5 minutes. Take out the bay leaves and stir the mushrooms and onion into the sauce. Cook for 2-3 minutes until the veggies are warmed through. (see note)
Remove the skillet from the heat and stir in the sour cream. Check the seasoning and add salt and pepper as needed.
Spoon the beef stroganoff over the cooked egg noddle sor stir the noodles right into the sauce. Sprinkle with chopped parsley for garnish. Serve warm.
Notes
Searing the beef adds a nice flavor to this dish and partially cooked the beef. The meat will continue to cook as the sauce simmers, but will most likely still be a bit pink inside. It's steak so that's okay. If you prefer your steak cooked through, you can sear it longer.
If you want the sauce thicker, combine 1 tablespoon of cornstarch with 1 tablespoon of water to make a slurry. Stir it into the sauce and simmer until it has thickened.