Bring a large pot of salted water to a boil. Add the wide egg noodles and cook according to package directions until just al dente. Drain and set aside, keeping warm.

While the noodles cook, pat the beef sirloin strips dry with paper towels. Season with about half of the kosher salt and half of the freshly ground black pepper, then sprinkle the all-purpose flour over the beef and toss to coat evenly.

Heat the olive oil in a large, wide skillet over medium-high heat. Add the floured beef strips in a single layer, working in batches if needed, and sear for 1–2 minutes per side until browned but not fully cooked through. Transfer the browned beef to a plate and set aside.

Reduce the heat to medium and add the unsalted butter to the same skillet. Once melted, add the thinly sliced yellow onion and cook, stirring often, for about 4–5 minutes until softened and lightly golden.

Add the sliced cremini mushrooms to the skillet with the onions. Cook for 5–6 minutes, stirring occasionally, until the mushrooms release their moisture and begin to brown.

Stir in the minced garlic, Dijon mustard, and sweet paprika. Cook for 30–60 seconds, stirring constantly, until fragrant.

Pour in the dry white wine and use a wooden spoon to scrape up any browned bits from the bottom of the skillet. Let the wine simmer for 2–3 minutes until it is reduced by about half.

Add the low-sodium beef broth and the remaining kosher salt and black pepper. Stir to combine, then bring the mixture to a gentle simmer.

Return the browned beef strips and any accumulated juices to the skillet. Reduce the heat to low and simmer for 4–5 minutes, just until the beef is cooked through and tender.

Turn the heat to very low. Place the full-fat sour cream in a small bowl and whisk in a few spoonfuls of the hot sauce from the skillet to gently warm it. Then stir the warmed sour cream into the skillet until the sauce is smooth and creamy. Do not let it boil.

Add the cooked egg noodles to the skillet or divide them among serving plates and spoon the beef stroganoff sauce over the top. Sprinkle with the chopped fresh parsley just before serving.

Serve the beef stroganoff with sour cream immediately while hot and creamy, making sure each portion has a generous amount of beef, mushrooms, and sauce.