Prepare the Beef:
Heat 1 tablespoon of olive oil in a large skillet over medium-high heat.
Season the beef strips with salt and pepper. Add them to the skillet in batches to avoid overcrowding. Sear until browned on both sides, about 2-3 minutes per side. Remove and set aside.
Sauté the Vegetables:
In the same skillet, heat the remaining oil or butter.
Add the onions and cook until softened, about 3-4 minutes.
Stir in the garlic and mushrooms, cooking until the mushrooms are tender and browned, about 5 minutes.
Make the Sauce:
Sprinkle the flour over the onion and mushroom mixture, stirring to coat evenly. Cook for 1 minute to remove the raw flour taste.
Slowly pour in the white wine or 1/2 cup of beef broth, whisking to deglaze the pan.
Add the remaining beef broth, Dijon mustard, and smoked paprika, whisking until smooth.
Simmer the sauce for 5-7 minutes, allowing it to thicken slightly.
Combine and Serve:
Return the beef strips to the skillet, along with any juices. Stir to coat in the sauce.
Reduce the heat to low and stir in the sour cream. Heat gently without boiling to prevent the sour cream from curdling.
Adjust seasoning with salt and pepper as needed.
Serve over buttered egg noodles, rice, or mashed potatoes. Garnish with fresh parsley.