In a small bowl, combine 4 tsp garam masala, 3 tsp chilli powder, 2 tsp ground coriander, cumin, paprika and turmeric and 1.5 tsp salt. In a medium bowl, combine the diced chicken with half the spices (save the rest of the spices for later).
Heat up around 1 tbsp veg oil in a pan over high heat. Once it just begins to smoke, space out your chicken (work in 2 batches if needed). Leave to fry until it begins to char, then flip and char the second side. You're not looking to cook the chicken at this point, this is purely to lock in the spices and caramelize the outside (a little char = extra and authentic flavour).
Remove chicken from pan and place into slow cooker. Lower heat to medium and add 2 tbsp butter. Add onion and fry until it begins to soften, then add ginger and garlic and fry for a minute or so longer. Scrape everything into the slow cooker.
Into the slow cooker pour in passata, water, tomato puree, your remaining spices and sugar. Give it a good stir then pop on the lid and cook on HIGH for 3-4hours or LOW for 5-6hours.
Remove the lid and flick the cooker to 'warm'. Allow the curry to cool slightly, or at least stop rapidly bubbling then gently stir in the cream. It's important the cream is at room temp and the curry cools slightly or the cream may curdle. Don't be too vigorous when stirring otherwise the chicken will break apart.
Serve with coriander/cilantro, rice and naan!