Heat the oven to Gas Mark 6, 200°C (400°F).
To cook the pork: Place the onion and carrots in the base of a roasting tin. Place the pork on top.
Mix the dry spice rub ingredients together and then rub all over the pork. Drizzle over a little oil.
Place the roasting tin in the hot oven and roast, uncovered, for 30 minutes. Then reduce heat to Gas Mark 4, 180°C (350°F). Cover the meat with tin foil and roast for a further 2 hours.
When the pork is cooked remove it from the oven and place on a large plate or board. Cover with tin foil and allow to rest while you make the gravy.
To make the gravy: Remove the large vegetables from the roasting tin. Strain off some of the fat then stir the flour into the remaining juices. Place over a medium heat and continue stirring with a wooden spoon. Gradually add in the stock or water and bring to the boil. Simmer for a couple of minutes, taste and season. Then strain through a sieve into a serving jug.
To serve: Slice the pork and serve with the gravy, mashed potatoes and roasted seasonal vegetable which can be cooked in the oven at the same time as the pork.