Pat the beef cubes very dry with paper towels and season them evenly with about half of the salt and half of the black pepper.
Heat 2 tablespoons of the olive oil in a large heavy-bottomed pot over medium-high heat until shimmering but not smoking.
Add about half of the beef in an even layer and sear on all sides until deeply browned, then transfer the browned beef to a bowl and repeat with the remaining beef, adding the remaining olive oil as needed.
Reduce the heat to medium, add the butter to the pot, and once melted stir in the onion and celery, scraping up any browned bits from the bottom as the vegetables soften.
Cook the onion and celery, stirring occasionally, until translucent and beginning to turn golden at the edges, then add the garlic and cook just until fragrant.
Pour in the dry red wine, if using, and simmer for a few minutes while stirring to deglaze the pot and reduce the liquid slightly.
Add the beef broth, water, Worcestershire sauce, tomato paste, dried thyme, dried rosemary, remaining salt, remaining black pepper, and bay leaves, stirring until the tomato paste is fully dissolved.
Return the seared beef and any accumulated juices to the pot, bring the mixture just to a boil, then reduce the heat to low to maintain a gentle simmer.
Cover the pot with a lid slightly ajar and simmer the stew gently for about 60 minutes, stirring occasionally, until the beef is starting to become tender.
Stir in the turnip chunks and mushrooms, re-cover the pot partially, and continue simmering for another 30 to 40 minutes, or until the beef is very tender and the turnips are soft but not falling apart.
Taste the broth and adjust the seasoning with additional salt and pepper as needed, then remove and discard the bay leaves.
Ladle a few tablespoons of hot broth from the pot into a small bowl, sprinkle the xanthan gum over the broth while whisking constantly, and whisk until completely smooth with no clumps.
Pour the xanthan gum mixture back into the simmering stew in a thin stream while stirring, then continue to simmer for a few minutes until the stew thickens to your desired consistency, adding a pinch more xanthan gum if needed and whisked into additional broth first.
Stir in the chopped parsley and let the stew rest off the heat for about 5 minutes to allow the flavors to meld and the sauce to set slightly.
Spoon the hearty keto beef stew into warm bowls, garnish with extra parsley if desired, and serve on its own or alongside a simple green salad or cauliflower mash.