1. Preheat the oven to 200C/400F/Gas Mark 6.
2. On a large roasting tin or deep tray arrange large chunks of the carrots, garlic and onion to form a trivet to raise the meat off the roasting tray.
3. Lay the leg of lamb on top of the vegetables. Using a sharp knife make a number of incisions to the fat of the lamb and stick sprigs of the rosemary in the fat.
4. Season lightly with a little salt and pepper and drizzle with a little oil and roast in the oven for 1-1½ hours (15 minutes per 1lb/450g for rare and 20 minutes for medium) turning over half way through.
5. Allow the meat to rest for at least 15 - 20 minutes before carving, this allows the juices to redistribute throughout the meat and therefore it will be far more tender and juicy to eat.
To make the Sauce:
1.Drain off the majority of the fat off the tray and put the tray on the stove top.
2. Sprinkle in the flour and add tomato puree and stir with a whisk until well coloured.
3. Gradually pour in the red wine and stock and continue to whisk until the mixture comes to the boil.
4. Sieve into a clean saucepan to remove any impurities and Add the chopped rosemary and simmer for 4-5 minutes until reduced and thickened nicely.
5. Adjust the seasoning and pour into a sauce boat to accompany the lamb.
Serve with some roast potatoes and seasonal vegetables.