Preheat the oven to 180C/160C Fan/Gas 5. Line an 8" square baking tray.
If making your own, make the pastry. Combine the butter and flour, either using your fingers to 'rub together' or by placing in a food processor and combining until the mixture resembles breadcrumbs.
Add the water until the mixture comes together to form a ball. Chill 10 mins in the fridge while the oven gets hot.
In a bowl, combine all the streusel ingredients using a fork to form a sand-like texture.
Roll the pastry on a lightly floured surface to about 1/2 inch thick. Place into the lined tin, pushing into all the corners. Spread over the mincemeat in an even layer. Sprinkle over the streusel topping.
Bake for 15-20 minutes, until the pastry and streusel are crisp and golden. Remove and serve warm with ice cream or let cool!