Prepare the Beef: Sprinkle salt and pepper over the thinly sliced beef. In a large skillet or frying pan, warm 1 tablespoon of oil over medium-high heat. Cook the beef strips in batches for approximately 2-3 minutes per side until they are nicely browned. Once cooked, transfer the beef to a plate and set aside.

Sauté the Vegetables: In the same skillet, pour in the leftover tablespoon of oil. Add the diced onion and sauté for 3-4 minutes until it becomes soft and translucent. Then, stir in the minced garlic, cooking for another 30 seconds while stirring constantly to prevent it from burning.

Cook the Mushrooms: Add the sliced mushrooms to the skillet and cook for 4-5 minutes, or until they release their moisture and become golden brown.

Thicken the Sauce (Optional): Dust the mushroom mixture with flour and mix thoroughly to ensure it’s well incorporated. Allow it to cook for 1 minute to remove the raw taste of the flour. This step aids in thickening the sauce, but you can skip it if you prefer a lighter consistency.

Add the Liquids: Add the beef broth, Worcestershire sauce, and Dijon mustard into the mixture. Stir well, scraping up any browned bits from the bottom of the skillet. Bring the mixture to a simmer and cook for 2-3 minutes, allowing the sauce to thicken slightly.

Add the Cream: Reduce the heat to low, then add the heavy cream or sour cream to the skillet. Stir to combine, then return the beef strips to the pan. Let everything simmer together for 5 minutes, or until the beef is tender and the flavors have melded.

Serve and Garnish: Season with additional salt and pepper to taste, then garnish with freshly chopped parsley. Serve the stroganoff hot over egg noodles, rice, or mashed potatoes.