Heat the oil and butter in a large non-stick casserole pot or large saucepan.
Add the onions and cook for 2-3 minutes. Add the garlic, ginger, chilli, cardamom, cumin, turmeric, garam masala and ground coriander. Cook over a medium heat until the onion is soft, being careful not to burn the spices.
Add the potatoes and squash and cook until the potato begins to stick to the bottom of the pan slightly. Add the stock and bring to the boil. Season, to taste, with salt and freshly ground black pepper. Reduce the heat and simmer for 10-15 minutes, until the potatoes and butternut squash are tender.
Stir in the yoghurt, cream and lemon juice. Add the cooked turkey, fold in and simmer to heat through. Sprinkle with coriander leaves and serve immediately.