Toss the butternut squash with the flaxseed oil on a baking sheet and season with salt and pepper. Spread out in a single layer and roast in a preheated oven, 200°C (400°F), Gas Mark 6, for 30 minutes until tender and beginning to char in places.
Meanwhile, mix the walnuts and maple syrup together in a small bowl and spread out over a separate baking sheet lined with foil. Sprinkle with the salt flakes and bake in the oven for 5–7 minutes until toasted and caramelized.
Put all the dressing ingredients in a screw-top jar and season with salt and pepper. Screw on the lid and shake well to mix.
Transfer the roasted squash to a salad bowl and add the spinach. Pour over the dressing and toss gently together. Sprinkle over the caramelized walnuts.