Pat the beef dry with paper towels. Season with salt, pepper, and garlic powder, pressing the seasoning into the meat.
Heat the oil in a large skillet over medium-high heat.
Sear the beef in batches until browned on at least two sides, about 2–3 minutes per side. Don’t overcrowd the pan.
Transfer the browned beef to the crockpot. Add the onion and garlic to the same skillet and cook 2–3 minutes, scraping up browned bits. Pour in 1/2 cup of the broth to deglaze, then pour everything into the crockpot.
Add the remaining broth, tomato paste, Worcestershire, thyme, rosemary, and bay leaf.
Stir to combine.
Stir in the turnip, rutabaga, celery, and mushrooms. Hold the zucchini for later so it doesn’t get mushy.
Cook on Low for 7–8 hours or High for 3.5–4.5 hours, until the beef is very tender and the root veggies are soft.
About 30 minutes before serving, stir in the zucchini and butter. Taste and adjust seasoning with more salt and pepper if needed.
If you want a thicker stew, sprinkle in xanthan gum a little at a time (start with 1/4 teaspoon), whisking well to avoid clumps. Let it sit for 5 minutes to set before adding more.
Remove the bay leaf.
Ladle into bowls and garnish with fresh parsley.