Pat beef slices dry and season with salt and pepper.
Heat 1 tbsp oil and 1 tbsp butter in a large skillet over high heat.
Quickly sear beef in batches for 30–60 seconds per side; remove and set aside.
Reduce heat to medium; add remaining oil and butter to pan.
Sauté sliced onions until softened, about 4 minutes.
Add mushrooms and cook until they release liquid and brown, 6–8 minutes.
Stir in garlic for 30 seconds, then sprinkle flour and cook 1 minute to remove raw taste.
Gradually whisk in beef broth, scraping up browned bits to make a smooth sauce.
Simmer sauce 3–4 minutes until it thickens slightly, then stir in Dijon mustard and Worcestershire sauce.
Remove pan from heat; stir in sour cream and lemon juice, adjust seasoning.
Return beef and any juices to the pan just to warm through; avoid boiling to prevent curdling.
Serve immediately over cooked egg noodles and garnish with chopped parsley.