In a heavy-based pan, heat 1 tablespoon of olive oil over medium-high heat. Toss the diced beef in flour seasoned with salt and pepper, then brown the beef in batches for about 5-7 minutes until golden all over. Remove the browned beef and set it aside on a plate while you prepare the vegetables, ensuring the pot is left with fond for flavor.

Lower the heat to medium and add the remaining tablespoon of olive oil to the same pan. Add the finely sliced onion and diced celery, sautéing for 5 minutes until they soften. Next, stir in the chopped carrots and parsnips, cooking for another 2-3 minutes until the vegetables start to caramelize, before returning the browned beef to the pan.

Pour in the Guinness and beef stock, scraping the bottom of the pan to incorporate all the flavorful bits. Stir in the Worcestershire sauce, soft light brown sugar, bay leaves, and fresh thyme. This mixture should bubble gently as it combines, infusing the stew with richness.

Cover the pot with a lid and reduce the heat to low, allowing the stew to simmer gently for 1 to 1.5 hours. Stir occasionally to prevent sticking, and add mushrooms during the last hour if you're using them. The beef will become tender, and the gravy will thicken beautifully, creating a hearty Beef and Guinness Stew that is bursting with flavor.

Once the beef is tender and the gravy is thickened, taste the stew for seasoning. Adjust with additional salt and pepper as needed. Remove the bay leaves and thyme sprigs before serving. Your comforting Beef and Guinness Stew is now ready to warm your heart on a chilly evening!