Season the beef strips with salt and pepper.
In a large skillet or Dutch oven, melt butter with olive oil over medium-high heat. Sear the beef in batches until browned; remove and set aside.
Add onion to skillet and cook until softened. Stir in garlic and cook until fragrant.
Add mushrooms and cook until browned.
Sprinkle flour over mushrooms and cook for 1 minute.
Gradually whisk in beef broth, scraping up any browned bits. Bring to a simmer.
Stir in Dijon mustard and Worcestershire sauce. Reduce heat and simmer for 10-15 minutes.
Stir in sour cream off the heat. Season with salt and pepper.
Return beef to skillet and heat through.
Serve over noodles, rice, or mashed potatoes. Garnish with parsley or dill.