• Preheat the oven to 150°C fan bake.
• Place the lamb into a roasting dish. Season with salt and pepper, drizzle with oil, and sprinkle over the dried rosemary.
• Add the halved onions, garlic cloves, and optional fresh rosemary to the tray. Pour in ½ cup of water.
• Cover the lamb with baking paper, then seal tightly with foil. Place in the oven and cook for 2 hours. This helps keep the meat tender.
• After 2 hours, remove the foil and baking paper. Return the lamb to the oven and increase the heat to 180 °C fan bake. Bake uncovered for a further 30–45 minutes, or until the outside is browned and the meat is tender. The internal temperature needs to be around 70 degrees. If you like it more rare then take it out earlier. You would also roast any of the veggies you want during this time.
• Remove the lamb from the oven. Wrap in baking paper, then foil, and cover with a clean tea towel. Let it rest for at least 30 minutes before carving.

To Make the Gravy

• Remove and discard any rosemary sprigs from the roasting dish.
• Check the fat in the pan — you want about 4–5 Tbsp. If there’s not enough, add a little butter. If there’s too much, spoon some off.
• Place the roasting dish over medium heat (or transfer contents to a pan). Add the Oxo cubes and flour, whisking to combine. Cook for about a minute.
• Gradually add the warm water, whisking continuously until smooth.
• Simmer for a few minutes, stirring, until the gravy thickens. Season with salt and pepper to taste.