Preheat the oven at 180°C.
Slice the croissants in half length ways, place the tops to the side. With the bottom half of the croissants, cut into square piece and place them into the baking dish.
Place in the frozen blueberries and sprinkle over the grated nutmeg and ground cinnamon.
In a pan heat up the double cream and granulated sugar until it reaches just before boiling.
In a separate bowl whisk the eggs together. Gradually add the double cream mixture whilst continuing to whisk. If you add too much at once it will scramble the eggs and you will have to start again or run the mixture through a sieve.
Once you have mixed all of the heated liquid into the eggs, stir in the vanilla extract and pour over the croissant and blueberries. Place the tops of the croissant on top, pressing down slight so they absorb some of the custard.
Place in the oven and bake for 40 minutes.
Server either hot or allow to cool, then place it in the fridge if you are eating it cold.