Blueberry (& Raspberry) Crunch Cake

I have halved the quantities in this recipe by Sue Lawrence, as her original quantities makes a huge cake and since this is quite a rich and filling cake, a small piece is quite sufficient!

The polenta gives the cake a lovely crunchy topping and quite a unique flavour.

I have only ever tried this recipe with fresh berries but I'm sure frozen ones would work just as well. The combination of blueberries and raspberries is excellent, but when I recently made this I didn't have any raspberries because they were too expensive, so I substituted them for 1 gala apple cut into small dice and it gave a good result.
This is great served with the blueberry syrup (see pancakes).

• Oven shelf position: upper-middle
• Oven temperature: pre-heat oven to 350 degrees F/180 degrees C/Gas mark 4
• Baking time: 50 minutes to 1 hour
  
• You will also need a 20cm/8in springform cake tin, lightly buttered

Ingredients
100g Polenta - plus 1 teaspoon
250g Self raising flour, sifted
160g Golden caster sugar
Grated zest 1 unwaxed lemon (optional)
160g Cold unsalted butter, diced
1 Large free-range egg, lightly beaten
1 tablespoon lemon juice (optional)
125g Blueberries
125g Raspberries
12g Demerara sugar

1. Preheat the oven to 180C/Gas 4
2. Place the 100g of polenta, the flour and sugar in a large food processor and process for a few seconds. Add the lemon zest and butter and process again until the mixture resembles breadcrumbs (alternatively rub the butter into the dry ingredients by hand as if making a crumble mixture)
3. Add the beaten egg and lemon juice then process using the pulse button, or mix, very briefly, until just combined.
   
4. Tip about two-thirds of the mixture into the prepared tin, pressing down all over.
5. Sprinkle the teaspoon of polenta over the base, then scatter over the blueberries and raspberries (or whatever fruit you're using)
6. Sprinkle the demerara sugar over the top, then crumble the remaining cake mixture over to cover. Press down very lightly, then bake in the preheated oven for 50 minutes to 1 hour.
7. Cool on a wire rack, then remove from the tin.