To dry out mandarin skins, use a Stanley knife or filleting knife to gently cut away as much white pith as possible. Use a dehydrator, or simply place peels on a plate or chopping board in a warm, dry area in your home. I do mine near a heater, and the drying process can take 1-2 weeks. Every time I eat mandarin, I just add it to the drying collection. Once dry, I break them down into small pieces and then blitz in a spice grinder. Keep for up to 3 months in an airtight container.
To make crudo, thinly slice the kingfish and scatter on a plate. Lay the thinly shaved fennel over the sliced fish.
Combine the extra virgin olive oil, ponzu and mandarin segments in a medium bowl and season to taste with salt. Gently spoon the dressing over the fish and fennel.
Using a fine mesh sieve, dust over some mandarin dust over the plate, using caution, as it is a strong flavour.
Garnish the plate with some fennel fronds. Serve with crostini.
