Heat the oven to 220C/200C fan/gas 7. Tip the portobello mushrooms, onions, carrots, celery and garlic into a roasting tin, toss in the sunflower oil, then spread out into a single layer. Roast for 40 mins, undisturbed – the veg should be very slightly charred.
Drizzle over the agave, vinegar, tomato purée, soy and 2 tbsp water. Toss together until completely coated, then return to the oven for 5 mins until sticky and caramelised. Stir in the flour and dried mushrooms, and return to the oven for a final 10 mins.
If the tin is flameproof, put on the hob over a low heat, stir in the port and cook until you have a thick paste mixed in with all the ingredients. Pour over two-thirds of the stock, add the herbs, bring to a boil and cook for 10 mins. If the tin isn’t flameproof, drizzle over the port, stir to loosen all the burnt bits, then tip everything into a pan and continue with the stock and herbs.
Turn off the heat and mash everything with a potato masher to extract as much flavour as possible. Carefully pass the contents through a sieve set over another saucepan, pushing down to release all the liquid. Add the rest of the stock and simmer until the gravy is thickened and rich. Leave to cool. To serve, simply reheat. Will keep, chilled and covered for up to three days or frozen for up to two months.