Remove the lamb shoulder from the fridge and allow to come up to room temperature before cooking.
Preheat the oven at 200ºC/400ºF/gas 6.
Pick most of the rosemary leaves into a pestle and mortar and bash with a pinch of sea salt until broken down. Peel and add 2 cloves of garlic and bash again. Muddle in the English mustard and 2 tablespoons of olive oil.
Slash the lamb all over with a sharp knife, season with black pepper then rub the rosemary mixture all over the lamb, making sure to get into all the nooks and crannies.
Peel and thickly slice the onions, then place in a large deep roasting tray with the remaining rosemary and unpeeled garlic cloves. Drizzle with the balsamic vinegar and pour in 200ml of cold water.
Place the lamb shoulder on top and roast for 20 minutes.
Carefully remove the tray from the oven and cover tightly with a double layer of kitchen foil, then return to the oven, turn the heat down to 160ºC/315ºF/gas 2½, and cook for 4 hours, or until the meat pulls easily away from the bone.
Carefully remove the shoulder to a platter, cover loosely with the foil and leave to rest.
Skim the fat from the tray, pick out and discard the rosemary, and squeeze the garlic cloves out of their skins.
Place the tray on the hob over a medium heat, stir in the flour and cook for 2 minutes.
Add 500ml of boiling water and the wine (if using), and stir well. Bring to the boil, then reduce the heat to low and simmer for 15 minutes, or until thick and glossy.
To make the root veg mash, peel and roughly chop the carrots and peel two cloves of garlic. Place in a large pan of boiling salted water over a medium-high heat and cook for 5 minutes.
Meanwhile peel and roughly chop the potatoes and parsnips, add them to the pan of boiling water with the carrots then cook for a further 15 minutes or until tender.
Drain in a colander and leave to steam dry. Tip back into the pan, add a lug of extra virgin olive oil, then mash with a potato masher. Season to taste.
Shred up the lamb and serve up with the mash and onion gravy. Delicious served with steamed seasonal greens.