Put the coriander stems and garlic in a food processor, pulse until finely chopped, add the turkey and 1 tsp of the fish sauce, and blend again. Roll into meatballs about 3cm in diameter.
In a saucepan, heat the tom yum or curry paste with a splash of water for 2-3 mins. Pour in the coconut milk, stock, lime zest and juice, and remaining fish sauce. Bring to a simmer, add the meatballs and cook for 3 mins. Add the vegetables and cook for 5 mins more, or until the meatballs are cooked through.
Add the noodles to warm through just before serving. Season, pour into 2 bowls and top with coriander leaves.