Preheat oven to 425° F
Mix the cauliflower with 3 tablespoons of the olive oil, ½ tsp salt, and some black pepper. Spread out in a roasting pan and roast on the top oven rack for 25 to 35 minutes, until the cauliflower is crisp and parts of it have turned golden brown. Transfer to a large mixing bowl and set aside to cool.
Decrease the oven temperature to 325° F. Spread the hazelnuts on a baking sheet lined with parchment paper and roast for 17 minutes.
Allow the nuts to cool a little, then coarsely chop them and add to the cauliflower, along with the remaining oil and the rest of the ingredients. Stir, taste and season with salt and pepper accordingly.
Serve at room temperature.