Step 1: Pat the sliced beef strips very dry with paper towels. Season generously with 1/2 teaspoon of salt and 1/4 teaspoon of black pepper.
Step 2: Heat 1 tablespoon of olive oil in a large, heavy-bottomed skillet or Dutch oven over medium-high heat. Sear half of the beef until browned on all sides (2-3 minutes per side). Do not overcrowd the pan. Remove seared beef to a plate and repeat with remaining beef, adding more oil if needed. Set aside.
Step 3: Reduce heat to medium. Add the remaining 1 tablespoon of olive oil to the skillet. Add the sliced mushrooms and cook, stirring occasionally, until they release their liquid and are nicely browned (5-7 minutes).
Step 4: Add the chopped onion to the skillet with the mushrooms. Sauté for another 3-5 minutes until the onion is softened and translucent. Stir in the minced garlic and cook for 1 minute more until fragrant.
Step 5: Sprinkle the flour over the mushroom and onion mixture. Cook, stirring constantly, for 1-2 minutes to cook out the raw flour taste and create a roux.
Step 6: Pour in the beef broth and Dijon mustard, scraping up any browned bits from the bottom of the skillet. Bring to a simmer, then reduce heat to low and let cook for 5-7 minutes, or until the sauce has thickened slightly.
Step 7: Return the seared beef and any accumulated juices to the skillet. Stir to combine and let it gently warm through in the sauce for about 2-3 minutes.
Step 8: In a small bowl, whisk the sour cream until smooth. To prevent curdling, temper the sour cream by slowly whisking a ladleful of the hot sauce into it. Then, gently stir the tempered sour cream mixture into the skillet. Do not bring the sauce to a boil after adding the sour cream.
Step 9: Stir in the fresh chopped parsley. Taste and adjust seasonings with remaining salt and pepper, or more if desired.
Step 10: Serve immediately over hot, cooked egg noodles or your preferred base. Enjoy your ultimate beef stroganoff recipe!