1. Heat your oven to 180°C/Fan 160°C/Gas 4. Unwrap the lamb and pat it dry with kitchen paper. Place it on a board and make small cuts all over it with a small sharp knife.
2. Pull 2 cloves from the bulb of garlic, then peel and finely chop them. Pick the leaves off the thyme and rosemary sprigs. Add the herb leaves to the garlic and chop a few times together to combine them. Rub the garlicky mixture all over the lamb, working it into the little cuts. Season the lamb with salt and pepper and set aside.
3. Peel and finely slice the red onions, lay them in the bottom of a small roasting tin, and place the lamb joint on top. Halve the remaining garlic bulb around the middle and place the 2 halves in the tin. Crumble the stock cube into a heatproof jug and stir in 450ml boiling water to dissolve the stock cube. Pour the stock around the lamb.
4. Cover the roasting tin with a loose layer of foil, scrunching the foil to the edges of the tin to seal it (or pop on a lid if your roasting tin has one). Slide the tin into the oven and roast for 3½ hrs. (See below on our tips on how to cook the veg).
5. Take the roasting tin with the lamb out of the oven and remove the foil from the lamb. Turn the oven temperature up to 240°C/Fan 220°C/Gas 9. Return the lamb to the oven for 25 mins to brown it.
6. Remove the garlic bulb halves from the tin and squeeze the soft half-cloves out of them. Pop the squeezed garlic into a cold bowl and pour in all the juices from the tin, along with the onions (the cold bowl will help set the lamb fat quickly so you can scoop it out).
7. 15 mins before you want to serve the lamb, scoop 2 tbsp of fat out of the bowl of roasting tin juices and pop into a pan. Scoop the rest of the fat off the lamb (you can keep it in the fridge for a few days for roasting veg). Stir 2 tbsp flour into the fat and cook, stirring for 2-3 mins. Slowly whisk in the juices from the roast tin till they are smoothly combined. Add the onion and garlic to the pan. Bring to the boil, then turn the heat down to a simmer and bubble gently for 10 mins till the gravy thickens.
8. When the lamb has rested for 1 hr, slice it and serve with the rich gravy poured over the top.
9. For the potatoes: While the lamb slow roasts, scrub the potatoes and quarter them or slice them into 6 chunky pieces. Add them to a pan of boiling water and simmer for 10 mins to just start cooking them. Drain and set aside till the lamb comes out of the oven. Pop a tin with 2 tbsp oil - rapeseed or olive oil is great - into the hot oven for 10 mins so it's sizzling hot. Turn the oven down to 200°C/Fan 180°C/Gas 6. Add the potatoes to the oil and turn a few times to coat them. Return to the oven and roast for 40 mins till golden, shaking the pan once or twice.
10. For the carrots: Trim and scrub the carrots, then slice them into batons about as big as a little finger. Place them in a deep frying pan or wide saucepan and add 1 tbsp butter, 1 tbsp honey and 1 tbsp balsamic vinegar. Pour in enough water to come halfway up the carrots, season with a little salt and pepper, and bring to the boil. Turn the heat down and simmer for 15-20 mins, tossing the carrots occasionally, till the carrots are tender and the sauce has reduced and become a little sticky.
11. For the purple sprouting broccoli: Put a pan of salted water on to boil. Trim any dry ends off the purple spouting broccoli, then halve or chop any thick stems so you have florets about as big as an index finger. When it's boiling, drop in the psb and simmer for 2-3 mins till th florets just start to turn a darker green. Drain straight away then tip back into the pan, add a splash of olive oil and some salt and pepper and toss to mix. Warm a heavy-based frying pan or griddle pan and when it's hot, add the psb. Dry fry for 4-5 mins, turning, till charred and smoky. Pop on a serving dish and squeeze over a little lemon juice to serve.