For the mash, place the butter into a saucepan to melt. Add the onion and sweet potatoes. Cover and cook for ten minutes, or until softened. Add the parsley and chives and mash until smooth. Keep in a warm place.
For the sauce, place the vinegar, butter, honey, and beef stock into a saucepan. Once boiling reduce the heat to simmer, and let sauce thicken. After 5-6 minutes add the peas.
Season the steak, with sea salt and freshly ground black pepper. Heat a griddle pan on a high heat and add the steak and cook for 3-4 minutes on each side, or until cooked toyour liking.
To serve, place the steak on a plate, add the mash and drizzle with the sauce.