Place the apples, sugar and water in a medium sized saucepan and bring to a gentle boil. Add in the frozen berries and stir gently to incorporate them with the apples. Simmer on a low heat for 5-6 minutes until the fruit just starts to soften. Taste the fruit and add a little more sugar if you feel it is required. I normally like to leave it a little tart as the crumble topping is quite sweet.
Preheat the oven to 180C/350F/Gas Mark 4. Grease and line a 13x7 tray with parchment paper
Meanwhile prepare the crumble topping by sieving the flour into a large bowl. Add in the brown sugar and the butter and run the butter into the flour and sugar mixture with your fingertips until a fine consistency has been achieved. Add the flaxseed at this stage also.
Divide half crumble into the prepared tin and press down firmly with a potato masher. Pour the fruit mixture on top of this and top with the remaining crumble, again pressing down firmly.
Transfer to the preheated oven and bake for approximately 30-35 until the crumble topping is golden brown.