To make the dressing, put the cashew nuts in a bowl with cold water and leave them to soak in the fridge overnight. The next day, drain them well and place in a food processor or blender with the parsley, garlic, mustard, 1 tablespoon lemon juice and water.
Blitz until smooth, scraping down the sides every so often. If the dressing tastes a bit gritty, add a dash more water and blitz again. Season with salt and more lemon juice to your taste.
Preheat the oven to 200ºC/ Fan 180ºC/400ºF/Gas 6.
Put the squash and onion into a large roasting tray with the olive oil (if using) and about 2 tablespoons cold water. Season lightly with salt and pepper and mix well. Place in the oven and roast for 20–30 minutes, or until the squash is tender and starting to color at the edges. Set aside until warm or cold, as you prefer.
Meanwhile rinse the quinoa in a sieve under cold running water. Pour the boiling water into a saucepan, add the quinoa and cook gently for 12–15 minutes, until tender. Set aside until warm or cold, as you prefer.
While the quinoa cooks, remove and discard the kale stalks. Tear the leaves into bite-sized pieces. Throw them into a large bowl with a good squeeze of lemon juice, then use your hands to massage the leaves for a few minutes.
To serve, tumble the squash, onions, quinoa and kale together in a large serving dish. Spoon the dressing over the top.