Season the beef strips generously with salt and pepper. Heat oil in a large skillet over medium-high heat.

Working in batches, brown the beef strips for 2-3 minutes per side. Remove and set aside.

In the same pan, melt butter and add mushrooms. Cook until golden brown, about 5 minutes.

Add onions and garlic, cooking until softened, about 3-4 minutes.

Sprinkle flour over the vegetables and stir for 1 minute.

Gradually add beef broth, stirring constantly to prevent lumps.

Bring to a simmer and cook until sauce thickens, about 5 minutes.

Meanwhile, cook egg noodles according to package instructions.

Reduce heat to low and stir in sour cream and Dijon mustard.

Return beef to the pan and heat through, about 2-3 minutes.

Serve over hot egg noodles and garnish with parsley if desired.