In the same cast-iron skillet used to cook the steak, add butter and shallots. Cook shallots for a few minutes until translucent. Next, add the mushrooms and cook for a few more minutes.
Deglaze pan with Madeira and cook until reduced by half. Add cornstarch, beef stock and tomato paste. Cook until sauce is glossy and thickens slightly. Add salt and pepper to taste. Add parsley. Dip steak frite bites in sauce. Enjoy!