Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the butter in a large frying pan, add the onion and fry until softened.
Stir in the butternut squash, garlic, ginger and chilli and cook for 2–3 minutes. Mix in the curry paste and cook for 1 minute to release the curry flavour. Stir in the stock and bring to the boil.
Transfer the mixture to the slow cooker pot. Arrange the tomatoes, cut side uppermost, in a single layer on top of the squash, then sprinkle with the coconut and a little salt and pepper. Cover with the lid and cook on low for 5–6 hours or until the squash is tender and the tomatoes are soft but still holding their shape.
Spoon into bowls, sprinkle with roughly chopped coriander and serve with plain or pilau rice (see below) and naan bread.