Preheat oven to 150C/300F. Once ready, roast the hazelnuts for 20 minutes, check on them at the 10 minute mark and move them around. Remove from the oven and set aside.
White hazelnuts are roasting, prep/cook the other ingredients. Discard outer layer and end of the leek, wash dirt off and slice in to thin rounds. Wash and cut the cauliflower in to large floretsOver medium heat, add 1 tbsp of the oil and the leeks. Cook for 10 mins, stirring occasionally. They are ready when soft and lightly brown.While the leeks are cooking - steam the cauliflower until just soft. Set aside. Add all ingredients except garnish, with 1 tbsp oil, to a blender and blend until smooth.
Heat 2 teaspoons of remaining oil in a small frypan on medium heat - add the sage leaves and crisp them up without burning. Set aside.
Roughly chop hazelnuts for garnish. Re-heat soup, serve w/garnishes.