For the sauce:
Melt two-thirds of the butter in a saucepan over a medium-low heat. Add the shallots and cook for about 5 minutes, stirring occasionally. Add the garlic and continue to cook for another 2 to 3 minutes, until the shallots are soft and translucent. Add the wine, stock, mustard, rosemary, salt, pepper and sugar, and bring to a boil. Simmer over a medium heat for about 30 minutes, or until the liquid is reduced by about half.
Meanwhile, place the remaining butter into a small bowl. Add the flour and mix into a smooth paste.
Once the wine mixture has reduced, remove the rosemary sprigs and reduce the heat to low. Whisk in the flour and butter paste, incorporating one teaspoon at a time, then simmer for 2 to 3 minutes until the sauce has thickened. Set the saucepan aside.
For the beef:
Remove the beef from the refrigerator and allow to stand at room temperature for at least 1 hour before cooking.
Preheat the oven to 200°C.
Generously season the beef all over with salt and pepper. Heat the oil in a large, oven-proof pan over medium-high heat until very hot. Add the beef and brown for about 10 minutes on all sides. After turning to the last side, transfer the pan directly to the oven. Roast for about 15 minutes, until a meat thermometer inserted into the thickest part reads 50°C for medium-rare (the temperature will continue to rise between 2-8°C on resting). Transfer to a board or tray, cover loosely with foil, and allow to rest in a warm place for 10 to 15 minutes.
Meanwhile, place the roasting pan back on the stovetop, remove most of the fat, add the 1/4 cup beef stock, and heat until boiling. Use a wooden spoon to deglaze the pan, scraping up any brown bits from the bottom. Mix the stock into the Cabernet sauce and bring to a simmer.
Carve the beef into 1 cm slices. Serve with caramelised shallots, crispy rosemary potatoes, steamed broccolini, and a generous jug of Cabernet sauce on the side. The ultimate indulgence with a glass of Wolf Blass Black Label.