Heat the oil large skillet over high heat. Add beef strips in a single layer. Quickly sear the beef strips on both sides (steak overcooks really fast, so a minute or two is all you need). Remove from the pan with a slotted spoon, transfer to a plate, and set aside.
In the same skillet, melt 1 tablespoon of butter over medium-high heat. Once it has melted, add 1 large chopped onion and 1 lb of sliced brown mushrooms. Sauté until the onions are translucent and the mushrooms are cooked.
Add garlic, and cook another minute.
Sprinkle the flour over the mushroom mixture and stir well to combine. Cook for another minute or two to cook the flour.
Slowly pour in the beef stock, mixing constantly to prevent lumps. Add the Worcestershire sauce, parsley, dry mustard, and paprika, and stir to combine.
Reduce the heat to low and simmer the sauce for 5 to 10 minutes until it has thickened.
Stir in the sour cream and vinegar until well combined. Add the reserved steak to the skillet and cook for a few minutes until the steak is heated through.
Taste the stroganoff and adjust the seasoning with salt, black pepper, and additional Worcestershire sauce as needed.
Serve hot over egg noodles, rice, or mashed potatoes for a comforting and satisfying meal.
Notes
When searing meat, it's important to use an oil with a high smoke point, such as canola, vegetable, sunflower, or grapeseed oil, to prevent it from burning and smoking during cooking.
For extra flavor, marinate the steak strips in two tablespoons of soy sauce for 15 minutes before cooking. If you do this, be sure to dry well before searing and skip the seasoning.
Don't be afraid of a hot pan. To quickly sear the steak, you want a really hot pan. A quick sear will ensure the steak isn't overcooked.
Be sure to cook the flour for a minute or two after adding it to the pan to eliminate its starchy flavor.
Use full-fat sour cream. Low-fat and fat-free sour cream will liquefy when heated and will thin out the sauce.
Add sour cream off the heat. Boiling sour cream can cause the sauce to curdle.