Roll out the pastry as thin as you can without breaking it, preferably on a cool work surface such as marble or metal. Cut out rectangles about 10cm by 6cm and put them onto a baking tray. Chill these for 20 minutes.
Preheat the oven to 200 C / Gas 6.
Put the creme fraiche or whipping cream into a bowl with the icing sugar and gently fold it together till peaks form.
Bake the pastry for 5 minutes or until just turning slightly golden. Take them out and put them back into the fridge for about 3 minutes or until cooled.
Return the pastry to the oven and cook for another 3 minutes or until golden brown.
Allow the pastry pieces to cool whilst giving the cream mixture another stir.
Put a layer of cream mixture onto a pastry sheet. Top with a few raspberries and strawberries. Put another pastry sheet on top. Repeat so that you end up with three pastry sheets with two layers of cream and fruit in between.
Repeat with remaining ingredients.
Dust the millefeuille with icing sugar and keep chilled until serving.