Add the oil to a heavy bottom pan or dutch oven. Once the oil is warm, add the beef. Season with salt & pepper, then cook on medium high heat, until all sides are browned.
Remove the meat from the pan and set aside. Leave the oil in the pan.
Add the onions and carrots to the dutch oven, stirring constantly. Cook until soft and fragrant.
Add the garlic and cook for about a minute or until fragrant, then add the tomato paste, stirring to coat everything. Allow to cook for a minute longer, to cook off some of the acidity.
Add the Guinness & Worcestershire sauce and de-glaze the pan, scrapping the brown bits off of the bottom of the pan, then bring to a simmer.
Add the beef and chicken broth back to the pan, along with the thyme and bay leaves. Bring to a boil, then reduce to a simmer. Cover and let cook for an hour and a half.
Add the potatoes, if using, and turnips. Cover and allow to cook for an additional 30 minutes or until they're cooked through.
Remove the thyme & bay leaves. At this point, if the stew is still quite runny, mix some cornstarch with a bit of water, then add it to the stew until it thickens and is at the desired consistency.
Add the mushrooms and let stew for an additional 15 minutes.
Garnish with parsley and serve!