Peel the onion and finely dice. Add to a large casserole dish or deep cooking pan along with the vegetable oil and gently fry over medium-low heat until the onions become translucent.
Peel and cut the rest of the vegetables (carrots, parsnips and swede) into 1-inch chunks making sure they are all roughly the same size.
Boil the kettle and make up your stock if using a stock cube.
Cut the potatoes into large chunks, leaving the skins on.
Add all of the vegetables and potatoes to the casserole dish and give it a good mix. Add the flour and stir until everything is well coated.
Add the stock to the pan and bring to the boil. Add the thyme and black pepper. Reduce the heat to low and allow to simmer for 45 minutes.
Taste the vegetable stew. If it's a little sweet add the gravy granules and white wine vinegar and mix well. Add salt to taste if needed (stock can be quite salty so I tend to leave adding extra salt to the end of cooking).
Slide a knife into the vegetables to make sure they are cooked all the way through - swede usually takes the longest. If the stew is too thick you can add more stock, if it's too runny add more gravy granules.
Serve immediately, using the parsley to garnish. Enjoy!