In a skillet- Brown the beef, onion, salt, pepper, and rosemary until the beef is no longer pink and the onion is translucent. Add in the garlic and cook until fragrant, about 2-3 minutes. Drain and set aside.
Add the mushrooms to the skillet with a tablespoon of water, for moisture, cook about 5 minutes until partially softened.
Add the beef/onion/garlic mixture back to the skillet with the softened mushrooms.
Next sprinkle the flour evenly over the beef and stir until fully incorporated, and it completely coats the beef and mushrooms.
Add in the beef stock, balsalmic, and Worcestershire. Bring to a boil and then a low simmer 10-15 minutes (uncovered) until the gravy has begun to thicken. Taste to see if more salt and pepper is needed.
While the gravy mixture is simmering bring a pot of salted water to a boil and cook noodles according to package directions.
Turn the heat off the gravy, and add in the sour cream and parsley. Stir.
Serve the warm gravy on top of the noodles!