The quality of the meat is crucial in Stroganoff, which is why beef fillet (tenderloin) is traditionally used. The meat is quickly seared to keep it tender and juicy.
The sauce is defined by the combination of beef stock, tangy mustard, and sour cream. Never boil the sauce after adding sour cream to prevent it from curdling – only reheat it gently.
Cut the beef fillet into thin strips (julienne) about 1 cm wide and 5 cm long. Season with salt and pepper.
Heat a little butter and oil in a large pan over high heat. Sear the meat in two or three batches, quickly (1-2 minutes per batch), just until browned. The meat should not release liquid. Remove the seared meat from the pan and set aside.
Add the remaining butter/oil to the same pan. Sauté the sliced onion until soft (about 5 minutes).
Add the sliced mushrooms and sauté until the water evaporates and they turn brown. Season with salt and pepper.
Dust the mushrooms and onion with all-purpose flour and stir.
Gradually pour in the beef stock (or some white wine) while stirring constantly. Add the Dijon mustard and cook until the sauce thickens. Taste the sauce and add more salt/pepper if needed.
Remove the pan from the heat. Stir in the sour cream until the sauce is smooth. The sauce should not boil after adding the sour cream!
Return the seared beef to the sauce and only reheat it gently. Serve immediately with boiled wide noodles or mashed potatoes, sprinkled with fresh dill or parsley.