Heat the oil in a large frying pan and fry the onion, garlic and carrot for 3 minutes over a medium heat, until softened. Stir in the mushrooms, vinegar and yeast extract or miso paste and cook for a further 5–7 minutes.
Add the rice and stock, bring to the boil, then reduce the heat and simmer gently for 20 minutes, until the rice is cooked and sticky. Stir regularly and add more water, if needed.
Stir in the lentils and oats and set aside to cool. Once cooled, stir in the suet.
Tip the mixture onto a baking tray lined with greaseproof paper and work into a log shape, adding a drop of water if it is too dry and crumbly. Wrap the haggis with the paper, then add a layer of foil over the top to make it watertight.
Put the wrapped haggis log in a saucepan filled with about 2.5 cm/1in water. Bring to the boil then reduce to a simmer and cook for 1 hour until firm.
Unwrap the haggis and serve with a dollop each of neeps and tatties, plus a drizzle of gravy.