Coat the Flank Steak (1 kilogram) in All-Purpose Flour (to taste). In a pan with a bit of Olive Oil (to taste), fry the meat until golden brown. We are not going to fully cook the meat; we just want to make the skin crispy. Set aside.
Into the same pan, add the Garlic (1 clove) and Onion (1). Cook for 2-3 minutes.
Return the meat to the pot and add Red Wine (125 milliliter). Cook it for a few more minutes to cook off the alcohol.
Cover everything with water, then add the Beef Stock (to taste). Cook for 2 hours over high heat. Remove fat with a spoon or a skimmer.
Fry the Mushroom (250 gram).
Add mushrooms and Dijon Mustard (125 gram) to the pot once the meat has cooked. Cook for an additional 15 minutes. Season with Salt (to taste).
Serve in large bowls with other side dishes like potatoes if desired.