Preheat the oven to 200°C/400°F.
Place all the vegetables, herbs, chicken pieces and bacon in a large, deep roasting tray.
Pour olive oil and sprinkle salt & pepper over the mixture and stir.
Roast for 45 minutes to 1 hour, stirring halfway through.
Position the roasting pan directly on the stove and tilt the pan to ladle out the separated fat from the pan juices. Set the separated fat aside in a small bowl.
Add the wine to the roasting pan, scraping the bottom of the pan well and allow the wine to reduce for 5 minutes.
Mix in the cornflour and allow to cook for 1 to 2 minutes.
Add either chicken or beef stock and simmer it until it has reduced slightly for 10 to 15 minutes.
Over a low heat, use a potato masher to mash the roasted mixture, moving and scraping all the baked residue from the bottom of the pan and mixing it well.
Turn the heat off and sieve the mixture into a medium saucepan, mashing and pressing the vegetables and chicken pieces so that as much liquid is released.
Heat the strained gravy on medium heat.
Once the gravy has heated up, position the saucepan half on the stove ring and ladle the scum which will gradually move towards one side. This makes it easy to remove and discard the scum.
Once all the scum has been removed, continue to simmer until the gravy coats the back of a spoon.
Add the jam to balance the gravy flavour.
Taste and add more salt & pepper, if needed.