Start by peeling, seeding and chopping all the vegetables.
Take a heavy bottomed, non-stick casserole pot and heat oil and butter in it over medium high heat.
Add chopped onions to the pot, and sauté them for 2 to 3 minutes or until they are soft and translucent (but not brown).
Then, add garlic and ginger and cook everything for a minute. Next, add all the spices (chilli, cardamom, cumin, turmeric, garam masala and ground coriander) to the sautéed onion, ginger, and garlic, and cook over medium heat for another couple of minutes, taking care not to burn the spices.
Add the chopped potatoes and butternut squash to the pot, and fry for 2 to 3 minutes, or until the potatoes begin to stick to the pan, stirring occasionally.
Now, add the chicken or turkey stock to the pan and bring the mixture to a boil over medium high heat.
Season the mixture with salt and freshly ground black pepper, reduce the heat to low, and simmer the curry for 10 to 15 minutes, or until the potatoes and squash are cooked through.
Stir in the cream and yoghurt, and cook for a few more seconds before adding the lemon juice.
Now, add the roasted turkey pieces to the curry, toss carefully, and simmer for 2 to 3 minutes (or until heated through).
Remove the pan from the heat, sprinkle with finely chopped coriander or parsley leaves, and serve immediately.