Season beef with salt and pepper and stir to coat.
Add beef to hot skillet and cook for 1 minute on both sides. Remove onto a plate.
To the same skillet, add onion slices, garlic and mushrooms. Stir and cook for about 1 minute. Add butter and cook, stirring often, until the onions are translucent.
Sprinkle flour over vegetables and stir in until there is no flour visible. Cook for about 2 to 3 minutes, stirring constantly.
Drizzle in the beef stock and Worcestershire sauce. Stir until combined. Simmer on low until thick, about 10 to 12 minutes.
In the meantime, in a stock pot bring water to boil. Add salt and noodles. Cook until done, about 7 minutes. Drain, set aside and keep warm.
When vegetable sauce is thick, add back cooked beef and stir in. Add sour cream and stir until combined and the sauce is creamy. Remove from heat.
Serve sauce over noodles. Garnish with chopped green onions or parsley.
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Notes
Please note, that the nutrition value can vary depending on what product you use. The information below is an estimate.