In a large pot, melt the butter. Add the chopped onion and sauté for 2 minutes. Add the sliced mushrooms. Sauté for 3 minutes over medium heat.
Cut the beef fillet into thin slices. Add to the mushrooms and cook for a few minutes, turning all the meat pieces.
Add the white wine. Let it evaporate.
Add the cream. Season generously with salt and pepper. Mix and cook over low heat for 5 minutes.
Serve sprinkled with chopped green onions, with mashed potatoes.