Pat the brisket dry with paper towels, then generously rub salt over all sides.
Heat a cast iron skillet or large pan over medium-high heat and add the avocado oil. Once hot, sear the brisket on each side for 2-3 minutes until a deep golden crust forms. Remove and set aside.
In the same skillet, add sliced onions and cook for 2-3 minutes until translucent. Pour in vegetable broth to deglaze, scraping browned bits. Set aside.
Pour 3-5 cups water into a slow cooker to partially cover brisket. Add mushroom bouillon cubes and stir until dissolved. Add fresh thyme sprigs.
Rub minced garlic over brisket, then season with smoked paprika and dried thyme.
Place brisket fat-side up in slow cooker or covered Dutch oven/roasting pan. Add remaining spices, onion and broth mixture, baby potatoes, carrots, mushrooms, gluten-free soy sauce, and dried rosemary.
Cover and cook on low in slow cooker for 6-8 hours. Baste with broth every 1-2 hours.
Alternatively, preheat oven to 325°F (160°C). Cover Dutch oven or roasting pan tightly and bake 3.5 to 4.5 hours, basting every hour.
When fork-tender, remove brisket from liquid, shred or slice, and return to broth. Serve with vegetables and ladle broth over each portion.