Peel the onions, then roughly chop along with the carrots and celery. Add to a large pan over a medium heat with a splash of olive oil and the herbs. Fry for around 15 minutes, or until turning golden.
Add the sugar and continue to cook for a further 5 minutes, or until sticky and caramelised. Stir in the flour, then add the Marmite, tomato purée and vinegar. Pour in the stock, then bring to the boil. Reduce to a simmer for around 10 minutes, or until thickened and reduced. Pass it through a sieve into another large pan, using a spoon to squeeze out all those lovely flavours. Serve with vegan nut roast.