Prepare the ingredients by thinly slicing the beef sirloin into strips, slicing the onion and mushrooms, mincing the garlic, spiralizing the zucchini into noodles, and chopping the fresh parsley. Set everything within easy reach before you start cooking.

Season the sliced beef sirloin with about half of the kosher salt and half of the freshly ground black pepper, tossing the strips gently so the seasoning coats the meat evenly.

Heat the 2 tablespoons of olive oil in a large, heavy-bottomed skillet over medium-high heat. When the oil is hot and shimmering, add the seasoned beef in a single layer, working in batches if necessary to avoid overcrowding the pan.

Sear the beef strips for 2 to 3 minutes per side, just until they are browned on the outside but still slightly pink in the center. Transfer the browned beef to a plate and set aside, leaving the flavorful browned bits in the skillet.

Reduce the heat to medium and add the 2 tablespoons of unsalted butter to the same skillet. Once the butter has melted, add the sliced onion and cook, stirring often, until it becomes soft and translucent, about 4 to 5 minutes.

Add the minced garlic to the onions and cook for about 30 seconds, stirring constantly, until the garlic is fragrant but not browned.

Add the sliced cremini mushrooms to the skillet along with the remaining kosher salt and black pepper. Cook, stirring occasionally, until the mushrooms release their moisture and become tender and lightly browned, about 5 to 7 minutes.

Sprinkle the all-purpose flour evenly over the onion and mushroom mixture. Stir well so the flour coats the vegetables and absorbs the butter, then cook for 1 to 2 minutes to remove the raw flour taste.

Slowly pour in the beef broth while stirring constantly, scraping up any browned bits from the bottom of the skillet. Continue stirring until the liquid is smooth and begins to thicken into a sauce.

Stir in the Worcestershire sauce and Dijon mustard, mixing them thoroughly into the sauce so the flavors are evenly distributed.

Reduce the heat to low and let the sauce simmer gently for about 3 to 5 minutes, stirring occasionally, until it has thickened to a creamy consistency that coats the back of a spoon.

Add the sour cream to the skillet, stirring it into the sauce until it is fully incorporated and the mixture is smooth and creamy. Keep the heat low and avoid boiling to prevent the sour cream from curdling.

Return the seared beef and any accumulated juices on the plate to the skillet. Gently stir to coat the beef in the creamy sauce, then simmer over low heat for 3 to 5 minutes, just until the beef is heated through and cooked to your preferred doneness.

While the beef stroganoff simmers, heat the remaining 1 tablespoon of olive oil in a separate large skillet over medium heat. Add the spiralized zucchini noodles in an even layer.

Cook the zucchini noodles, tossing gently with tongs, for 2 to 4 minutes until they are just tender but still slightly crisp, being careful not to overcook so they do not become watery.

Taste the beef stroganoff sauce and adjust the seasoning with a little more kosher salt or black pepper if needed. Stir in most of the chopped fresh parsley, reserving a small amount for garnish.

To serve, divide the warm zucchini noodles among serving plates or shallow bowls, then spoon the creamy beef stroganoff over the top of each portion.

Garnish the plated beef stroganoff with the remaining chopped fresh parsley and serve immediately while hot and creamy.