Cut the beef fillet into fine strips. Melt 1 tablespoon butter in a pan and fry the strips of meat until light brown on all sides, season with salt and pepper, remove from the pan and keep warm.
Sauté the finely diced onion and the cleaned, sliced mushrooms in the remaining fat (2 tablespoon) in the pan. Pour in the meat stock and let the liquid boil down a little.
Mix the sour cream with the Dijon mustard and pour into the sauce. Bring to the boil, then season with creme fraiche, lemon juice, salt and pepper. Add the sliced pickle and strips of meat to the sauce. Warm up, but no longer boil.
Goes well with it: rice, ribbon noodles or parsley potatoes