Pat shanks dry, sprinkle with sea salt. Heat avocado oil in a Dutch oven. Sear each side until browned. Remove and set aside.
In the same pot, add onion. Sauté for 2-3 minutes until softened. Add carrots and celery and sauté for another 2–3 minutes.
Add ½ cup broth or water to loosen browned bits from the pot.
Stir in the remaining broth, nomato sauce, thyme, and bay leaves. Return shanks to the pot. They should be almost covered with liquid.
Cover and cook on low heat for 2 ½–3 hours (or in a 325°F / 160°C oven). Shanks are ready when fork-tender and marrow is soft.
Stir the tapioca slurry until smooth then drizzle into the simmering sauce while stirring. Let it bobble for 2-3 minutes.
Sprinkle parsley over just before serving to brighten the dish.
