Trim and slice the Beef Sirloin into thin strips against the grain; season lightly with Fine Salt and Black Pepper and toss with a tablespoon of All-Purpose Flour to give the meat a light coating.
Heat a large skillet over medium-high heat and add a mixture of Neutral Oil and Unsalted Butter so the butter doesn’t burn; when the fat is hot, sear the beef in a single layer until browned but not fully cooked (about 1–2 minutes per side). Remove the beef to a plate and set aside.
In the same pan, add a little more Unsalted Butter if needed and sauté the sliced Onion until softened and translucent, about 4–5 minutes.
Add the sliced White Button Mushrooms to the pan and cook until they release their liquid and begin to brown, another 5–7 minutes.
Stir in the Tomato Paste and Mustard, cook for 1 minute to develop their flavors, then sprinkle in a little extra All-Purpose Flour if you want a thicker sauce and cook for 30 seconds more.
Deglaze the pan with the Beef Stock, scraping up any browned bits from the bottom; bring to a gentle simmer and let the sauce reduce slightly for 3–5 minutes.
Return the seared Beef Sirloin and any accumulated juices to the pan and simmer just until the beef is cooked through, about 2–3 minutes depending on thickness.
Remove the pan from the heat and stir in the Sour Cream until the sauce is smooth and creamy—do this off the direct heat to prevent curdling. Taste and adjust seasoning with more Fine Salt and Black Pepper as needed.
Serve the stroganoff immediately over egg noodles, rice, or mashed potatoes, garnishing with a final grind of Black Pepper if you like.
