Toss beef with 1 tablespoon flour, salt & pepper. Shake off excess flour.
Heat olive oil over medium high heat in a saucepan. Brown beef in small batches until lightly browned, about 45 seconds on each side. Remove from pan and set aside.
Reduce heat to medium and add butter & onion. Cook until softened, about 4-5 minutes.
Add mushrooms & garlic and cook until softened, about 4 minutes more. Stir in 2 tablespoons of flour and cook 1 minute.
Add broth, Worcestershire sauce, dijon mustard, and thyme. Bring to a boil, reduce heat and simmer 5 minutes. If needed, you can thicken the sauce with a cornstarch slurry. Simply combine equal parts cornstarch and water and add to boiling sauce a little at a time. Stir constantly until sauce reaches desired thickness.
Add beef (with any juices). Cook 2 minutes more or just until heated through.
Stir a few Tablespoons of the sauce into 1/2 cup of sour cream to temper it so the sour cream doesn’t curdle then add it to the pan while stirring constantly.
Season with salt & pepper, serve over egg noodles, and garnish with parsley.