Prepare the barbecue for direct cooking over medium heat (180 to 230°C). Preheat the barbecue with a large Weber casserole dish (without lid) for 10 minutes. Alternatively, you can use a barbecue side burner.
Once the barbecue has preheated, add the butter or roast drippings, onion and garlic. Cook for 4 minutes, or until the onion has softened and started to brown, stirring as required.
Add the flour, thyme and season with salt and pepper. Cook for 2 minutes, stirring every minute.
Add the truffle and wine. Cook for 3 minutes, to cook off the alcohol, stirring as required.
Add the stock and cook for a further 12 to 15 minutes or until the liquid has reduced by a third.
Strain through a sieve and serve immediately.