Heat the oven to 240C (220C fan)/gas mark 9. Mix the squash wedges with two tablespoons of the oil, two teaspoons of salt and a good grind of black pepper. Spread out on a large, parchment-lined baking tray and roast for 35-40 minutes, until golden-brown. Set aside to cool.
Turn down the oven to 170C (150C fan)/gas mark 3. Put the tomato halves skin-side down on a parchment-lined baking tray, sprinkle with a quarter teaspoon of salt, drizzle with the remaining tablespoon of oil and roast for 80 minutes, until softened.
Put the ginger, chilli, garlic, sugar and a quarter teaspoon of salt in a medium bowl. Mix to a paste, then spoon on top of the tomatoes and cook for 40 minutes more, until caramelised, then set aside to cool.
Put all the ingredients for the lime yoghurt in a small bowl, add half a teaspoon of salt and a good grind of pepper, mix well and put in the fridge until ready to serve.
Spread out the squash on a large platter and arrange the tomatoes in between. Drizzle over the yoghurt, sprinkle over the coriander, cashews and shallots, if using, and serve.