Rub the lamb shoulder all over with salt and pepper. Set aside to marinate at room temperature for 1 hour.
Preheat the oven to 140° C (285° F).
Peel and cut the onions in halves and place them on a large and deep baking tray along with rosemary sprigs and garlic.
Place the lamb shoulder on top of the flavorings and add 1 liter of water to the tray.
Cook the lamb for 4 hours, or it falls apart easily when touched with a fork.
Place the lamb into a large bowl and cover it with tin foil, and let it rest somewhere warm for 30 minutes.
While resting the lamb, start preparing the gravy.
Remove as much fat as you can from the oven tray, add the ½ liter of stock and place it on medium heat.
Deglaze the pan, making sure you scrape all the brown goodness into the stock.
Let it boil for 10 minutes, remove from the heat and then pass it through a fine chinois or a sieve into a small saucepan. Press firmly on the vegetables to squeeze all the juice out of them.
Place the saucepan on medium-low heat and let the stock simmer for another 15 minutes.
Prepare the slurry by mixing 2 tablespoons of cornflour with 3 tablespoons of cold water.
Slowly whisk the slurry into the gravy. Let it simmer for a couple of minutes then remove the pan from the heat.
Taste the gravy and add seasoning, if necessary. You rarely would need extra seasoning.