Leave the raisins to soak in a bit of warm water for a couple of minutes. Grease a round springform pan or cake tin with olive oil or coconut oil. Preheat the oven to 200 degrees celsius (390 F)
Put the chestnut flour in a bowl, preferably with a sieve to prevent lumps from forming. Add the water little by little, stirring constantly with a spatula, again to prevent lumps from forming. You can also do this step with a food processor, blender or mixer.
Remove the raisins from the water, dry them off a little bit and add 2/3 of them to the dough.
Cut especially bigger walnuts and hazelnuts in little pieces and add 2/3 of the them to your dough.
Put the dough into a grased tin. Distribute the raisins and nuts you have left and rosemary on top of the dough.
Bake your castagnaccio for about 30 minutes. When it's ready, you will see typical cracks on top of the castagnaccio. Depending on your oven and the thickness of your dough it can be that your castagnaccio will need 5-10 minutes more.