For the bacon sauce, put the butter in a pot and melt it, sauté the bacon in it (the fat from the bacon should be translucent). Add the grated onion and sauté very briefly. Sprinkle flour on top and mix well.
Gradually add the beef broth while stirring and mix to a smooth sauce without lumps. Bring the sauce to a boil briefly and let it steep in the reheat for 5 minutes.
Season the bacon sauce with a little salt (bacon is already salty) and pepper.