Melt the butter in a pan. Add the flour and braise it to a golden color. Stir nonstop to prevent it from burning.
Pour the meat broth in a thin trickle, add the salt, black pepper to taste. Separate the egg yolk from the white.
Mix the egg yolk with the mustard, sour cream and lemon juice. Beat in a blender on low.
Pour this into the pan and leave on the stove on low briefly to thicken.
Serve the sauce with your steaks.