Preheat the oven to 180 ° C top / bottom heat. Grease an ovenproof baking dish with butter. Mix the oatmeal, buckwheat flour, coconut blossom sugar, cinnamon and salt. Add the softened butter and knead into crumble. Roughly chop the hazelnuts and fold in. Chill the streusel until you are ready to use it.
Peel, quarter and core the apples and cut into small pieces. Distribute the apple pieces in the baking dish. Sprinkle the crumbles evenly over the top and press down a little.
Bake the crumble in the preheated oven on the middle rack for about 25 minutes until the crumble is golden brown. Serve lukewarm.
Note: The crumble is particularly crispy thanks to the chopped hazelnuts, which I mix with the oat flake crumble before baking. I prepare the Apple Crumble with butter, coconut oil or vegetable margarine are good for the vegan variant.