Sear the Beef
Season beef with salt and pepper. Heat oil in a large skillet over high heat. Sear beef quickly in batches until browned. Remove and set aside.
Cook Aromatics
Lower heat. Add butter, onion, and mushrooms. Cook until mushrooms are golden and onions soft. Add garlic and cook briefly.
Build the Sauce
Pour in beef stock and let reduce slightly. Stir in Dijon mustard and cream. Simmer gently.
Finish
Return beef to the pan. Warm through, then remove from heat and fold in sour cream. Adjust seasoning.
Serve
Garnish with parsley and serve immediately over noodles or potatoes.