Heat the olive oil and dairy free margarine in a large, oven-proof casserole pan or saucepan. Add the onion, leek and celery and fry gently for about 10 minutes until soft and translucent.
Add the crushed garlic and fry for thirty seconds then stir in the flour.
Stir in the dry white wine (if using) a little at a time; don't add it all in one go or you will end up with a lumpy sauce. Once you have incorporated all of the wine, gradually stir in the stock.
Add the pearl barley along with the thyme, bay leaves and rosemary. Bring up to the boil; allow to boil rapidly for 10 minutes, stirring occasionally.
Turn down to a gentle simmer then add the carrots, parsnips, potato and swede.
Cover the pot and simmer gently for 30-40 minutes until the pearl barley and vegetables are soft. Stir the stew regularly to prevent it from catching on the bottom. Top it up with a little extra stock if needed but bear in mind that this stew is meant to be quite thick.
While the stew is cooking preheat the oven to 200C/400F/gas mark 6 and prepare the dumplings.
Mix together the self-raising flour, salt and dried herbs in a large bowl. Add the cold dairy free margarine and rub it in using your fingertips until the mixture resembles fine breadcrumbs. Gradually stir in enough cold water to bring the mixture together into a ball.
Once the vegetables and pearl barley are tender, remove the pan from the heat. Remove the thyme and rosemary sprigs (if using fresh herbs) and the bay leaves. Season generously to taste with salt and pepper.
Roll the dough into balls about the size of a golf ball and dot them all over the top of the stew. Place the uncovered pan in the oven and bake for about 20-25 minutes until the dumplings are golden.
The stew is best eaten fresh, but leftovers will keep in the fridge for up to 5 days (the stew will thicken as it sits).