Heat oven to 220C/200C fan/gas 7. Tip the turkey bits and/or chicken wings into a sturdy roasting tin with the onions, carrot, celery and garlic. Toss in the oil and spread out into a single layer. Place in the oven for 40 mins undisturbed – you want them the right side of just burnt, as this will give you lots of flavour.
Remove the tin from the oven and mix in the honey, soy sauce and tomato purée. Toss everything together until all the bits are completely coated, then return to the oven for 10 mins until sticky and caramelised. Remove the tin from the oven again, sprinkle over the flour and dried mushrooms (if using) and return to the oven for a final 10 mins.
If your roasting tin is flameproof, put it on a low flame, add the vinegar and sizzle for a moment. Pour in the Port and cook until you have a thick, glutinous paste mixed in with all the ingredients – it will look quite messy! Add the stock and herbs, bring to the boil and cook for 10 mins. (If your tin isn’t flameproof, add the vinegar, stir to loosen all the burnt bits from the tin, then tip into a saucepan to continue.)
Turn off the heat and use a potato masher to mash everything to extract as much flavour out of it as you can. Carefully pass the contents through a sieve over another saucepan, pushing down on the contents of the sieve. Simmer the gravy until thick and glossy, then leave to cool. Can be chilled for up to 3 days or frozen for up to 2 months. Reheat the gravy and serve as it is, or add to the turkey roasting juices for even more flavour.