Petits - Beurre Bars

I like traybakes and bars and wanted to make something with blueberries. I found this recipe in an old French magazine and it used raspberries, so I thought I'd try it with blueberries and it worked very well. The French recipe uses Petits - Beurre biscuits and I found some in Sainsburys. Waitrose and Tesco also have them apparently, but shortbread or other plain biscuits could be used.

Preheat oven 180C/gas4                        Grease and line a 20cm square cake tin.

You need: for the topping:
175g plain flour, 150g of crushed Petits- beurre or biscuits of choice, 60g caster sugar, 50g of chopped walnuts, 150g soft butter,
for the filling:
 225g blueberries, 50g caster sugar, 3 tbspn water, 3 tbspn flour, juice of 1/2 a lemon

For the topping, mix the flour, biscuit crumbs, sugar and nuts together in a large bowl. Rub in the butter to get a crumbly mixture. Put half of this into the bottom of the cake tin and press down well. Bake for 10 mins in the oven then take out.
For the filling, put the blueberries, sugar and 2 tbspn of water into a pan; bring to the boil and and cook for 2 mins. Add the flour, 1 tbspn water and the lemon juice and mix well till thickened.
Spoon the fruit mixture over the baked crumb mixture then put the other half of the crumb mixture on top. Mark it into bars with a knife. Put it back in the oven and bake for 20 mins.
Cool on a wire rack then cut into bars.

A lovely crunchy topping, then the soft blueberries and then another crunchy layer. This would work well with apples or pears. A nice alternative to flapjacks.