Wash the tapioca well. Cut the tapioca roots roughly into 3 pieces. Place the tapioca pieces n a pressure cooker, add a pinch of salt and sufficient water, and cook on high for about 3 whistles.
Once the pressure releases, drain the water. Peel and cube the tapioca. Set aside.
Meanwhile, finely chop onion, ginger and green chilli.
Heat 1 tablespoon oil in a pan over medium heat.
Sizzle mustard seeds, curry leaves and dry red chilli.
Add the chopped onions, green chili, minced ginger to the hot oil. Sauté until the onions turn translucent and fragrant.
Add the boiled and drained tapioca cubes to the pan. Sprinkle turmeric powder and salt. Gently mix to coat the tapioca with the spices and onion mixture.
Cook the tapioca on medium heat, stirring occasionally, for about 2-3 minutes. This allows the flavors to meld.
Sprinkle the grated coconut over the tapioca and stir to combine. Cook for an additional 2-3 minutes.
Garnish with coriander leaves. Remove from heat.
Serve Tapioca Stir Fry with white rice and rasam.