Begin by slicing the beef into thin strips. This helps ensure that the meat cooks quickly and stays tender.
Season the beef with a pinch of salt and pepper.
Heat 1 tablespoon of olive oil in a large skillet over medium-high heat.
Add the beef slices in batches (avoid overcrowding) and sear them for about 1-2 minutes per side. You want a nice brown crust on the outside while keeping the inside juicy.
Once browned, transfer the beef to a plate and set aside.
In the same skillet, melt the butter over medium heat.
Add the chopped onions and sauté until they become soft and translucent (about 3-4 minutes).
Add the minced garlic and mushrooms, cooking for an additional 4-5 minutes until the mushrooms have released their moisture and are golden.
Stir in the flour to help thicken the sauce, and cook for about 1 minute.
Gradually pour in the beef broth, stirring to combine. Then, mix in the Dijon mustard and Worcestershire sauce.
Let the sauce simmer for 5-7 minutes until it thickens slightly.
Lower the heat and stir in the sour cream, making sure it’s fully incorporated. Don’t let the sauce boil once the sour cream is added to avoid curdling.
Add the cooked beef back into the pan and simmer for another 3-5 minutes, allowing the flavors to meld.
Taste the stroganoff and adjust seasoning with salt and pepper if necessary.
Serve the beef stroganoff over a bed of egg noodles, mashed potatoes, or rice. Garnish with freshly chopped parsley for a pop of color and freshness.