If roasting the squash (see note above), preheat the oven to 200°C. In a small bowl, combine all the spices under 'Panch Phoran'.
In a large bowl, toss the squash cubes with the 1 tbsp ghee/oil and a light sprinkling of salt. Spread out on two large baking trays lined with foil so that the cubes are in a single layer. Bake for 10 minutes, then stir and spread out again. Bake for 10 - 15 more minutes until the cubes are cooked through and begin to char in places.
Meanwhile, place a heavy, preferably cast iron pan over low heat. Add the remaining 2 tbsp of ghee/oil and when hot, add the panch phoran mix, the bay leaf and the red chillies. Stir and fry gently for about 30 -40 seconds until you can smell the spices. You want them to go a shade darker, no more.
Add the onions, increase heat to medium and saute, stirring regularly, until they turn golden. Now add the garlic & ginger, stir and fry for about 2 or 3 minutes.
Add the pureed tomatoes/passata, turmeric powder and stir. Increase the heat and stir frequently until all the liquid from the tomatoes evaporates and the mixture has an oily glaze. Season with salt and chilli powder to taste.
If not roasting the butternut squash, add it to the pan now with some salt and a splash of water (I do realise that I just asked you to lose all the moisture in the previous step, but the frying of the tomato paste is necessary for the flavour to develop), cover and cook, stirring once or twice until the vegetable is cooked through.
If roasting the butternut squash, add it to the pan with any juices left in the tray. Stir well and fry for a minute or two. Turn off the heat, taste and add lemon juice as needed. If making it sweet and sour, add sugar to taste. Mix well and serve garnished with chopped coriander leaves.