Place the steaks on a cutting board, spread 1 tablespoon of mustard on each side and cover with shallots. Season with salt and pepper. Carefully turn the steaks from both sides in the flour and fry in butter and oil (first with the shallots side down) for 2-3 minutes on each side.
Boil the potatoes in salted water and cut them in half. Cut the fennel into fine slices. Puree the garlic, parsley and oil until smooth, add salt and pepper. Mix the parsley oil with the potatoes and fennel.
Serve the steaks still warm with the parsley potato salad.