Preheat the oven to 190°C (170fan/ Gas 5/ 375F) and grease and line a 20cm (8 inch) square baking tin with nonstick baking paper or parchment paper.
Put the flour, oats, and cinnamon in a large mixing bowl and rub in the cold cubed butter until you get a breadcrumb consistency. You can do this in the food processor if you prefer, using the pulse setting.
200 g Plain Flour, 200 g Oats, 1 teaspoon Cinnamon, 200 g Butter
Stir in the sugar.
75 g Light Brown Sugar
Press half of the crumble mixture into the base of the tin. Compact it down firmly with the back of a heavy metal spoon.
Spread over the mincemeat.
400 g Mincemeat
Mix the chopped walnuts into the remaining crumble mixture and scatter on top of the mincemeat. Lightly press down with your hands.
75 g Walnuts
Bake in the oven for 35-40 minutes until golden brown.
Allow to cool completely before cutting into bars. Dust with icing sugar if you like!