Slice the white onion into thin feathers, halve the mushrooms, chop the fresh parsley, and grate the Parmesan cheese if needed.

Heat a large skillet over medium-high heat and sear the 1 lb beef loin goulash on both sides until browned.

Transfer the browned beef loin goulash from the skillet to a plate and reserve it.

Add the sliced white onion, 1 tsp salt, 1 tsp paprika, and 1/2 tsp black pepper to the same skillet.

Sauté the seasoned onion over medium heat for 6 minutes, stirring occasionally, until soft and translucent.

Add 2 cups mushrooms, 1 tbsp dried oregano, 2 tbsp butter, 1/4 cup soy sauce, and 1 tbsp mustard sauce to the skillet.

Cook the mushroom mixture for 3 minutes over medium heat, stirring until the butter melts and the mushrooms begin to soften.

Pour 1/2 cup white wine into the skillet and scrape the browned bits from the pan.

Simmer the wine mixture until the alcohol aroma dissipates and the liquid reduces slightly, about 2 minutes.

Stir 1 cup heavy cream and 1/4 cup Parmesan cheese into the skillet until the sauce is smooth.

Return the reserved beef loin goulash and add 1/4 cup chopped fresh parsley to the creamy sauce.

Cook for 3 to 4 minutes over medium-low heat, stirring gently, until the beef is heated through and the sauce lightly thickens.

Serve the beef stroganoff hot with the mushroom cream sauce.