Grease or spray the inside of your slow cooker with cooking spray.
Toast whole spices in a large frying pan on a low heat until they smell fragrant - be careful not to burn your spices. Grind your toasted spices to a powder.
Cook the diced onion with 2 tablespoons of oil in a covered jug or bowl in the microwave on full power for 4 minutes.
Add the garlic, chilli and ginger to the cooked onions and cook for a further minute on full power.
Whizz up the onion mixture with a stick blender to a smooth puree.
Heat a tablespoon of oil in your large frying pan and add the beef chunks - brown well before transferring to the slow cooker. (You may have to do this in batches)
Return the frying pan to a gentle heat. Add the freshly ground spices, Madras Powder or Paste, Tamarind paste, tomato puree, salt and pepper and then cook for 2 minutes.
Add in the onion, garlic and chilli puree, passata and water and turn up the heat, stirring constantly.
Bring to a gentle simmer before tranferring your Madras Sauce to the slow cooker.
Cook on low for 8 hours or high for 4 hours (low setting is best)
Serve with fresh coriander, sliced chillies and rice.