To make the vegan gravy, heat the oil in a heavy based pot or pan on medium heat and add the chopped onion, celery and carrot. Sauté for around 5-7 minutes or until the onion is tender and beginning to brown at the edges a little.
Stir in the flour and cook for two minutes, continuously stirring. Don't rush this step, we need that flour to dissolve. Add the smashed garlic.
Add the red wine to deglaze - give the base of the pot a good scrape with your wooden spoon to pick up all those cooked-on flavours - and then stir in the Marmite and tomato paste. The mixture with be gluggy, don't worry, it is supposed to be.
Add the fresh herbs (thyme, sage and bay leaf).
Stir in the vegetable stock and continue cooking for a further few minutes, stirring regularly, until the gravy begins to thicken. Reduce the heat to low and cook for 5-minutes, stirring occasionally.
Go ahead and add the butter and turn the heat to low. Cook for a further 3-4 minutes.
Remove from the heat and add a splash of vinegar and season with the salt and pepper.
Allow the gravy to sit for 5-minutes to continue infusing the flavours before straining into a gravy jug. Press the veggies into the sieve to get as much liquid out as you can. If the gravy is too thick, add a splash of water to thin it out. Return to the pan to warm if needed before serving.