Preheat the oven to 200°C/392°F (without fan).
Place the onions, carrots, celery, bacon, bay leaves, sage, and rosemary into a large, high-sided roasting dish. The roasting dish should be oven-proof and also suitable for using on the stove later.
Using a very sharp knife, break the chicken wings so that the bones are cracked in places, but the wings are still intact.
Place the chicken wings on top of the other ingredients in the roasting dish.
Drizzle with olive oil and season generously with sea salt and freshly ground pepper.
Roast for 1 hour or until the chicken wings are golden and nicely browned. The more colour the chicken wings have, the more colour and flavour your gravy will have.
Remove the roasting dish from the oven and place it on the stove over medium-high heat.
Sprinkle over the flour and let it cook for several minutes.
Add the sherry or white wine, and let it bubble away for a few minutes until most of it has evaporated.
Add enough boiling water to just cover the ingredients (about 1.5-2 litres or 6-8 cups).
Simmer over medium heat for about 30-45 minutes, topping up with boiling water as necessary to maintain the same level of liquid.
During this time, use a potato masher to squash and mash everything together, and to extract as much flavour as you can from the ingredients. Towards the end of the cooking time, all of the vegetables will have broken down and the chicken wings will have also fallen apart.
Once everything in the pan looks like mush, strain everything through a fine sieve into a large bowl or jug, extracting as much liquid as you can from the ingredients by pressing down with a wooden spoon. What you will have left is the gravy stock.
If there is a lot of fat on the surface of the gravy stock, skim this off and discard.
Let the gravy stock cool before storing in the fridge or freezer.