Pat the beef slices dry with a paper towel and season with salt, pepper, and smoked paprika (if using).
In a large skillet, heat 1 tablespoon of butter and 1 tablespoon of olive oil over medium-high heat.
Sear the beef in batches for about 1-2 minutes per side until browned. Remove and set aside.
Sauté the Vegetables:
In the same skillet, add the remaining 1 tablespoon of butter.
Add onions and mushrooms, cooking for 4-5 minutes until softened.
Stir in garlic and cook for 30 seconds until fragrant.
Make the Sauce:
Pour in beef broth, Worcestershire sauce, and Dijon mustard, stirring to combine.
Simmer for 5 minutes, allowing the flavors to develop.
Thicken the Sauce:
In a small bowl, mix 1 tablespoon cornstarch with 2 tablespoons water to create a slurry.
Stir it into the sauce and let it simmer for 2-3 minutes until slightly thickened.
Add Sour Cream and Beef:
Lower the heat and stir in sour cream, mixing until smooth.
Return the beef to the pan and cook for another 2-3 minutes, just until heated through. Avoid overcooking to keep the meat tender.
Serve and Garnish:
Serve hot over egg noodles, mashed potatoes, or rice.
Garnish with fresh parsley and enjoy!