Preheat the oven to 425 F. Place the cauliflower on a parchment-lined baking sheet, drizzle with 1-3 tablespoons of olive oil, and toss with some salt and pepper. Roast in the oven for 25-35 minutes until parts of it are turning golden brown. Transfer to a large mixing bowl and set aside to cool down.
Decrease the oven temperature to 325 F. Spread the hazelnuts on a baking sheet lined with parchment paper (I used the same one as the cauliflower, just switched the paper) and roast for 17 minutes (although depending on your oven, I suggest watching these closely, mine were burnt at 17 minutes, and I took my second try out around 10 minutes).
In a small bowl, whisk together the remaining 2 tablespoons olive oil, the sherry vinegar, maple syrup, cinnamon, and allspice. Set aside.
Allow the nuts to cool a little, then coarsely chop them and add to the cauliflower, along with the pomegranate seeds, celery, parsley, and the dressing. Stir, taste, and season with salt and pepper accordingly.