In a heavy saucepan or Dutch oven, heat 1 tablespoon of olive oil over medium-high heat. Add the diced beef in batches, ensuring not to overcrowd the pan. Sear for about 5–7 minutes until browned on all sides, then remove the beef and set it aside on a plate.

Reduce the heat to medium and add the remaining tablespoon of olive oil. Once hot, add the chopped onion and finely diced celery, sautéing for about 5 minutes until they soften and become translucent. Next, stir in the peeled and chopped carrots and parsnips, cooking for an additional 2–3 minutes until they start to soften.

Return the browned beef to the pot with the sautéed vegetables. Pour in the Guinness, followed by the beef stock, Worcestershire sauce, and light brown sugar. Toss in the bay leaves and fresh thyme. Stir well to combine all the ingredients.

Cover the saucepan and reduce the heat to low, allowing the mixture to simmer gently for about 1 to 1½ hours. Check occasionally, stirring to prevent sticking.

After the initial simmering period, add the optional mushrooms if using. Stir them into the stew and cover the pot again. Continue cooking for another 1 to 1½ hours, or until the beef is tender.

Once the beef is wonderfully tender, taste and adjust seasoning with sea salt and pepper as needed. Remove the bay leaves before serving and ladle the stew into bowls.