Start by trimming and pounding the rib-eye steak into thin slices. Slice the cremini mushrooms and chop the onion finely. Organize your ingredients for easy access.

Heat a large pan over medium-high heat and add a splash of neutral oil. Sear the rib-eye steak slices for 2-3 minutes on each side until browned. Remove from the pan.

In the same pan, reduce heat to medium and add 2 tablespoons of butter. Once melted, sauté the chopped onion and sliced mushrooms for 4-5 minutes until softened.

Sprinkle plain flour over the vegetables and mix to create a roux, cooking for a minute. Gradually add the low-salt beef broth, whisking in the Dijon mustard.

Remove from heat and fold in the full-fat sour cream. Return the seared beef to the pan, mixing gently. Season with kosher salt and black pepper.

Serve hot over egg noodles or tagliatelle, garnished with chopped chives.