Cut the beef into 1-inch chunks and set aside on a plate. Dice the onion into 1/2-inch pieces, cut carrots and celery into 1/2-inch pieces, mince the garlic cloves, and chop fresh parsley for garnish. Measure out the Worcestershire sauce, Guinness, beef broth, rosemary, thyme, bay leaves, salt, and pepper into small bowls or cups. Sift the xanthan gum through a fine mesh strainer to break up any lumps—this prevents it from clumping when added to the hot stew.
Heat 2 tablespoons of olive oil in a large heavy-bottomed pot or Dutch oven over medium-high heat until shimmering. Working in batches to avoid overcrowding, brown the beef chunks on all sides until they develop a rich golden-brown crust—this typically takes 2-3 minutes per batch. Don't stir constantly; let each side caramelize before turning. Transfer the browned beef to a clean plate and set aside.
In the same pot with the rendered beef fat, add the diced onion and cook for 5 minutes, stirring occasionally, until softened and translucent. Add the minced garlic and cook for another 30 seconds until fragrant. Deglaze the pot by pouring in the Worcestershire sauce and scraping up any browned bits stuck to the bottom—these flavorful browned bits add incredible depth to your stew.
Return the seared beef to the pot along with any accumulated juices. Pour in the Guinness stout and beef broth, then add the carrots, celery, root vegetables, rosemary, thyme, and bay leaves. Stir well to combine all ingredients, ensuring the vegetables are partially submerged. Bring the mixture to a simmer on the stovetop, then cover and transfer to a 325°F oven for 3 hours. The beef should become fork-tender and the flavors will meld beautifully.
Remove the stew from the oven and place it on the stovetop over medium heat. Slowly sprinkle in the sifted xanthan gum while stirring constantly—I add it gradually and stir vigorously to prevent lumps from forming. Simmer for 2-3 minutes until the stew reaches your desired thickness. Taste and adjust seasoning with the measured salt and black pepper, adding more if needed.
Ladle the rich, hearty stew into bowls and top with fresh chopped parsley. The stew is best served immediately while it's hot and fragrant.