Put the berries in a plastic bowl and let them defrost while you prepare everything else.
Peel the apples, take out the core and chop them in blocks, not too small as you don't want them to cook away during the baking process.
Mix the apples and the berries in the bowl with a spoon and let sit till the crumble is ready.
Preheat the oven on 185 degrees with hot air, otherwise 200 degrees top and bottom heat.
Mix the butter with the oatmeal, the flour and the sugar in the mixer for 5 minutes on highest power, add then the hazelnuts and the cinnamon, mix for 2 more minutes.
Strain the berry-apple mix through a sieve and save the juice for the sauce. After straining the fruits put them on the bottom of the oven form and spread them evenly.
Take the crumble dough and spread it all over the fruit mix in the form. The crumble should not be thicker than 0.5 cm on top to bake evenly. If you have crumble dough left, you can easily freeze it away for next time.
Sprinkle the sugar over the crumble mix and put the form into the oven, bake for around 30 minutes till the crumble is golden brown.
Take 2 tablespoons from the juice and dissolve the Maizena in there. Pour the rest of the juice into the sauce pan and add sugar, bring to simmer on medium heat. When the juice is nearly boiling, add the Maizena,reduce heat and let thicken.
Take the crumble out of the oven and let it cool down for 10 min.