Allow the steak to sit at room temperature for 30 minutes to 1 hour. Pat dry with paper towel. Pound the steak once and season with sugar, let sit for 20 minutes.
Preheat the oven to 200 degrees C.
Place the potatoes in a baking dish, drizzle the melted butter and sprinkle with parsley. Bake in the middle of the oven until evenly browned and soft inside; about 30 minutes.
Before you cook the steak, wipe off the sugar with a paper towel. Season with salt and pepper.
Heat a frying pan with oil over high heat. Pan fry the steak until one side is brown; about 2-3 minutes. Turn the steak over and cook until the other side is evenly brown; 2-3 minutes. The cooking time depends on how well done you like your steak. Adjust the timing accordingly.
Set the steak onto a warm plate and allow to rest for 5 minutes.
Wipe the frying pan clean with a paper towel. Heat the pan over high heat. Add the butter and garlic then fry until aromatic and browned, 2-3 minutes.
Stir fry the onions until soft, 2-3 minutes. Add the chicken stock, red wine and cornflour slurry. Allow to thicken then serve with the potatoes and steak.