Pat the Beef Sirloin dry and spread a thin layer of German Medium-Hot Mustard over both sides; season with Salt and freshly ground Black Pepper.
Lightly dredge each slice in All-Purpose Flour, shaking off excess to ensure an even, thin coating.
Heat a mixture of Clarified Butter and Butter in a large skillet over medium-high heat. When hot, add the beef and sear quickly, about 1–2 minutes per side, until well browned; transfer the meat to a plate and tent loosely to keep warm.
Reduce heat to medium and add a bit more Butter if needed. Add the sliced Onion and cook, stirring occasionally, until soft and beginning to caramelize, about 6–8 minutes.
Stir in the Tomato Paste and cook for 1 minute to deepen its flavor, then pour in the Red Wine to deglaze the pan, scraping up any browned bits from the bottom. Let the wine reduce by about half.
Pour in the Beef Stock and bring to a gentle simmer. Season to taste with more Salt and Black Pepper if needed.
Mix the Cornstarch with a little cold Water to make a smooth slurry, then whisk it into the simmering sauce to thicken until glossy and slightly reduced.
Return the seared beef to the pan, spoon the sauce over the slices, and simmer gently for 2–4 minutes more until the meat reaches your desired doneness and the flavors meld. Finish with a small knob of Butter whisked in for shine and richness.
Serve the rostbraten hot with plenty of pan sauce spooned over the meat. Optional: add a final grind of Black Pepper and a light sprinkle of Salt to taste.
