First prepare the pickled red onion - for best results do this at least a couple of hours in advance or the day before. Add the sliced onions, coriander and mustard seeds to a container. Add the vinegar, water, salt and sugar to a pan on medium heat. Once the sugar and salt has dissolved remove from the heat and pour over the onions. Once cooled, seal and keep in the fridge.
Now for the squash. First, pre-heat your oven to 220°C (200°C fan). In a medium-sized bowl mix together the olive oil, harissa paste and salt. Toss the cubed butternut squash in it until fully coated and evenly space out on a foil-lined baking tray. Cook for 20-25 mins until a knife easily pierces through.
Add the bulgur wheat, stock and harissa paste to a pot on medium heat. Give everything a stir and cover, cooking until all the liquid has absorbed and you can fluff up with a fork. Add the lemon juice, mix and taste for seasoning - depending on your stock you may need to add salt. Add the bulgur to a large bowl and let cool for about five minutes.
To the bowl with bulgur add the squash, feta, spinach, and about a quarter of the onions (the rest can be used for other meals and will keep in the fridge for several weeks). Mix everything together and serve, optionally topping with mixed seeds and/or coriander.