Position the rack in the centre of the oven and preheat to 200°C or 180°C fan.
In a small bowl, mix the pepper, salt, rosemary and thyme. Brush the mustard all over the beef, then coat beef completely with the herb mixture. Place beef on a roasting rack set in a roasting pan. Roast for 1 hr 15 mins, or until a meat thermometer inserted into the centre of the beef registers 55-60°C for medium-rare, or 60-65°C for medium. The beef will continue to cook and the internal temperature will continue to rise as it rests, so don’t overcook it. Transfer beef to a carving board and rest for 15 mins before carving.
Meanwhile, to make the caramelized onion jus, heat a heavy-based medium saucepan over medium heat. Add the butter and cook for 4 mins, or until the butter melts and turns a golden brown color. Add the onions, season with salt and pepper, and cook, stirring often, for 15 mins, or until the onions are translucent and begin to brown. Add the thyme and continue cooking, stirring often, for 15 mins, or until the onions have become a deep brown color. Stir in the flour. Add the vinegar and cook for 30 secs. Stir in the stock and bring to a simmer. Simmer for 5 mins, or until reduced slightly. Season to taste with salt and pepper. Remove from heat, cover to keep warm, and set aside until the beef is ready.
To make the smashed potatoes, place potatoes in a large saucepan and add enough cold water to cover them by 2.5cm. Add 1 tablespoon of salt and bring to a boil over medium-high heat. Reduce heat to medium-low and simmer for 9-10 mins, or until potatoes are just tender. Drain the potatoes and set aside to cool slightly. Using your hand, gently press each warm potato on a cutting board until it just barely “cracks” and is slightly flattened on each side. Don’t use too much force, or the potatoes will fall apart.
Heat a large, heavy-based frying pan over medium heat. Add the oil, then add potatoes and cook for 5 mins, or until golden brown on the bottom. Turn the potatoes over and add butter. Tilt the pan to move the melted butter around. Cook for 5 mins, or until potatoes are golden brown on the bottom. Season with salt and pepper, and add the parsley.
Carve beef into thick slices and serve with the smashed potatoes and caramelized onion jus