In a dry frying pan over a medium heat, toast the cumin and coriander seeds for 2 minutes, stirring often, until they start to smoke then remove from the heat. Set aside on a cool plate.
Melt the butter in a pan over a medium heat then add chillies, onions, and garlic and fry for 6 to 8 minutes until they start to turn translucent.
Add the cumin and coriander seeds along with the ginger and cook and stir for 1 to 2 minutes. Add the powdered spices and salt and keep stirring to prevent it sticking and burning on the base of the pan. Pour in the passata and water then reduce the heat and allow to simmer for 10 minutes.
Blend in a food processor or use a handheld liquidiser to blitz until smooth. Return to the pan if needed and simmer for a further 5 minutes. Serve hot.
Instead of the passata, you can substitute with 500g of chopped tomatoes.