Season the beef cubes with salt and black pepper, then brown in batches in hot avocado oil in a Dutch oven over medium-high heat, turning until well browned on multiple sides. Avoid crowding so the meat sears rather than steams; transfer browned beef to a plate.

Cook the diced onion for 3 minutes until softened, then add the minced garlic and tomato paste and cook for 1 minute, stirring until fragrant. Scrape up browned bits from the bottom so they dissolve into the sauce.

Add the beef broth, Worcestershire sauce, dried thyme, bay leaf, and the browned beef, then bring the mixture to a simmer. Cover and simmer at a gentle bubble for 1 1/2 hours, stirring occasionally so the beef stays submerged.

Add the halved radishes, quartered cremini mushrooms, and sliced celery, then return to a simmer and cook covered for 30-40 minutes until the beef and radishes are tender. Keep a steady gentle bubble and adjust heat as needed.

Sprinkle in the xanthan gum while whisking constantly so it disperses smoothly, then simmer for 2 minutes to thicken. Stop whisking once the gravy turns glossy and coats the back of a spoon.

Remove the bay leaf and stir in the chopped fresh parsley, then taste and serve hot. Look for a thick, dark gravy with tender radishes and beef throughout.