Pour the red wine into a saucepan and bring to the boil. Simmer until reduced by approximately one third.
Add the roasted onions from beneath the lamb.
Stir the Gault's Lamb Stock Concentrate into the water, then pour into the saucepan and bring back to the boil.
Add the Worcestershire sauce, balsamic vinegar and a generous grind of cracked black pepper.
Combine the cornflour and cold water to make a smooth slurry.
Slowly whisk the cornflour slurry into the simmering gravy.
If you used Gault's Moroccan Seasoning on the lamb, add a healthy pinch to the gravy for an extra layer of flavour.
Simmer gently for 2 minutes until the gravy has thickened.
Stir through the chopped fresh parsley. For a glossy finish, add the cold butter and gently swirl the pan until melted.
Serve immediately over Roast Oyster Shoulder of Lamb or your favourite roast lamb cut.

Simon Says

Cook to temperature, not time.
Every oyster shoulder is a different size. A meat thermometer takes the guesswork out. Once it reaches 92 to 95°C, it'll be tender and still hold its shape when sliced.
Why I don't deglaze the roasting tray.
People often ask why I don't scrape all the juices out of the roasting tray. The answer is simple. When I roast oyster shoulder slowly at 160°C, what's left in the tray is mostly rendered lamb fat. I'd rather leave that behind and build the gravy with the roasted onions, red wine and Gault's Lamb Stock Concentrate instead. It gives a cleaner, richer tasting gravy.
Use a good balsamic.
You're only using a tablespoon, so it's worth using a good one. It gives the gravy a bit of lift without making it sweet.
Add the cornflour slurry slowly.
Whisk it in a little at a time and keep an eye on the consistency. You may not need all of the slurry. Once the gravy is as thick as you like it, let it simmer for a couple of minutes before serving.