Pat the beef dry very well with paper towels. Slice into thin strips, then season with the salt and black pepper. Sprinkle over the flour and toss lightly so the beef is very thinly coated. That light flour coating helps the beef brown a little better and later helps the sauce cling and thicken slightly without becoming heavy. Drying the beef thoroughly before this whole process even begins is what allows the flour to adhere cleanly and the meat to sear properly rather than steaming against any residual surface moisture.

Bring a large pot of salted water to a boil. Cook the egg noodles until just tender according to the package directions. Drain, then toss with the butter so they stay glossy and separate. Set aside. Do not overcook the noodles. They should stay soft but still hold their shape because they will sit under the sauce. Tossing the noodles with butter immediately after draining coats each strand before they have a chance to stick together, keeping them genuinely separate rather than clumped by the time they're plated.

Heat the oil and butter in a large skillet or sauté pan over high heat. Once hot, add the beef in batches and sear very quickly, just 45 to 90 seconds per batch, until lightly browned on the outside but not fully cooked through. Transfer the beef to a plate. Do not crowd the pan. If the beef is crowded, it will steam instead of sear, release too much liquid, and turn tougher. High heat and short cooking are the keys to tender stroganoff beef. That deliberately brief, hot sear is calibrated to build real colour on the outside while leaving the interior genuinely underdone — this dish's final stage depends on that headroom to finish the beef gently rather than overcooking it twice.

Lower the heat to medium-high. Add a touch more oil or butter if needed, then add the mushrooms. Cook them without stirring too often until they release their moisture and begin to brown properly, about 6 to 8 minutes. Add the onion and cook for another 4 to 5 minutes until softened. Stir in the garlic and cook for about 30 seconds. Let the mushrooms actually brown. This is where much of the deep savory flavor comes from. Pale mushrooms will give a flatter sauce. Stirring only occasionally, rather than constantly, gives the mushrooms genuine, sustained contact with the hot pan — that patience is exactly what allows them to move past simply releasing moisture and into actual browning, which constant stirring would interrupt before it ever really develops.

Add the Dijon mustard, Worcestershire sauce, paprika, and black pepper. Stir briefly. Sprinkle in the flour and stir for about 1 minute so it coats the vegetables and cooks slightly. Gradually pour in the beef stock while stirring, scraping up any browned bits from the bottom of the pan. Bring to a gentle simmer and cook for 4 to 6 minutes until the sauce thickens to a glossy, creamy brown consistency. Cooking the flour briefly in the fat and vegetables before any liquid is added, rather than adding it straight to the stock, is what cooks out its raw taste and lets it properly thicken the sauce once the stock does go in.

Lower the heat to very low. Stir in the sour cream until smooth. Return the beef and any accumulated juices to the pan. Stir gently and warm through for 1 to 2 minutes, just until the beef is heated and barely finished cooking. Stir in part of the parsley and taste for salt and pepper. After the sour cream goes in, do not boil the sauce hard. Gentle heat keeps it smooth and prevents splitting. Also, the beef should only return at the end so it stays tender and juicy. Sour cream's proteins are genuinely vulnerable to curdling under high heat, and keeping this final stage at very low heat is what protects the sauce's smooth emulsion while also giving the deliberately underdone beef exactly the brief, gentle finish it needs.

Divide the buttery egg noodles among 4 bowls. Spoon the beef stroganoff over the top. Finish each serving with a small spoonful of sour cream, chopped parsley, and freshly ground black pepper. Serving immediately captures the dish at its glossiest and the beef at its most tender, right before either the sauce or the meat has any chance to change texture as it sits.