Heat the oil in a large stock pot over medium high heat. Add the onion and cook for about 5 minutes, until it becomes fragrant and translucent.
Add the garlic, mushrooms, and a pinch of salt and continue to cook until the mushrooms release their liquid, about 5 to 10 minutes. Sprinkle the flour over the vegetables and stir to combine
Add the carrots, parsnips, and potatoes to the pot along with the stock, stout, vegan Worcestershire sauce, tomato paste, bay leaves, thyme and, rosemary. Bring the mixture to a boil and then reduce the heat and allow the stew to simmer. Allow the stew to simmer for 30 minutes, or until the vegetables are fork tender. Remove the bay leaves and then season with pepper and the remaining salt.