Allow the steak to sit at room temperature for 30 minutes to 1 hour. Pat dry with kitchen towels. Pound the steak once and season with sugar, let sit for 20 minutes.
Preheat the oven to 200 C / Gas 6.
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 kitchen towel. Season with salt and pepper.
Heat a frying pan with 1 tablespoon 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 kitchen towel. Heat the pan over high heat. Add the butter and garlic, 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 and serve with the potatoes and steak.