Cook black rice: Pressure cook soaked rice with 2 cups water and a pinch of salt for 5-6 whistles until completely soft.
Soak tamarind: Soak a marble-sized tamarind in 1 cup warm water. Extract and strain.
Make kuzhambu base: Heat 3 tbsp sesame oil. Add 1 tsp mustard seeds. When they pop add 10 sliced shallots and 5 garlic cloves. Fry 10 minutes until golden.
Add tomatoes and spices: Add 3 chopped tomatoes, 1.5 tsp red chilli powder and 1 tsp coriander powder. Cook 8 minutes.
Add tamarind water: Pour tamarind water. Add salt. Bring to a boil. Simmer 10 minutes.
Add cooked black rice: Add the soft cooked kavuni arisi to the kuzhambu. Stir well — the black rice thickens and colours the kuzhambu a beautiful deep purple.
Simmer together: Cook on medium heat 8-10 minutes until kuzhambu is thick and richly coloured.
Add tempering: Heat 1 tbsp sesame oil. Add curry leaves and dried red chilli. Pour over kuzhambu.
Serve: Adjust salt. Kavuni arisi kuzhambu has a unique earthy flavour and stunning purple colour. Serve with steamed rice.