Preheat the oven to Gas Mark 3, 160°C (320°F).
Place the shoulder of lamb into roasting tin. Cut slits in the meat and insert the garlic slivers into the slits. Mix the olive oil, lemon zest, juice and rosemary in a small bowl. Brush the mixture all over the lamb, making sure to get into all the nooks and crannies. Season with salt and pepper. Then add the onion, carrot and celery to the pan along with the water. Cover the roasting tin tightly with tin foil and let it stand at room temperature for 30 minutes, if you have time.
Roast the lamb in the preheated oven for 2 hours. Halfway through check the water level, if it has evaporated, add a little more. After 2 hours remove the tin foil and turn the heat up to Gas Mark 4, 180°C (350°F) and cook for a further 20 minutes.
The cauliflower can go into the oven at this stage. In a mixing bowl combine the cauliflower, shallot, olive oil, cumin, turmeric and seasoning and mix until evenly coated. Place seasoned cauliflower on baking sheet/roasting tin and roast in the oven with the lamb for 15 minutes or until lightly browned. When the lamb is cooked transfer it to a serving platter, cover loosely with foil, and let it rest while you make the gravy.
Place the roasting tin over a low heat. Stir in 1 tablespoon of flour and cook for two minutes. Gradually pour in 250ml stock, stirring all the time, until you have a thickened sauce. Stir in the honey then simmer for 2 minutes, using a wooden spoon to stir, scraping any sticky bits from the tin. Strain the gravy into a small saucepan, then simmer and season to taste. When you carve the lamb add any extra juices to the gravy.
To serve: Slice the lamb and serve it with the cauliflower, potatoes and gravy on the side.
