Season the Beef: Use paper towels to pat the beef dry, then sprinkle it evenly with salt and pepper to season.
Brown the Meat: Heat the avocado or olive oil in a large Dutch oven or heavy-bottomed pot over medium-high heat. Work in batches to sear the beef cubes for 2–3 minutes per side until browned all over. Transfer the seared beef to a plate and set aside.
Sauté the Onion: Add the onion chunks to the same pot, then pour in the apple cider vinegar. Use a wooden spoon to scrape up any browned bits from the bottom of the pot while cooking the onion for 3–4 minutes until softened.
Coat with Arrowroot: Return the browned beef to the pot with the onions. Sprinkle the arrowroot powder over the mixture and stir until the beef and onions are evenly coated and the powder is dissolved.
Build the Broth: Stir in the apple cider, beef broth, thyme, and bay leaf. Bring the mixture to a boil, then reduce the heat to low. Cover the pot partially with the lid and let it simmer for about 1 hour.
Add Vegetables: After the beef has simmered, add the chopped turnips, carrots, and celery to the pot. Continue cooking for another 20–30 minutes, or until the vegetables are tender when pierced with a fork.
Finish and Serve: Remove the bay leaf before serving. For an optional garnish, sprinkle fresh thyme sprigs on top, and enjoy the stew while warm.