Defrost the puff pastry ahead of time, if you have bought frozen pastry. This should take 2-3 hours at room temperature.
Unroll the first sheet and cut a 20cm circle out of it, then cut a 22cm circle from the second sheet. I used a side plate as a guide because I’m not confident enough to do it freehand. Put the circles of pastry on baking sheets lined with baking parchment, and put them in the fridge for an hour to chill.
After an hour preheat the oven to 200C (180C for a fan oven, Gas 6).
Beat the butter with a handheld mixer until it is light and fluffy, then beat in the icing sugar and almonds with a wooden spoon. At first it will look like there isn’t enough butter, but keep mixing and it will form a paste.
Beat one of the eggs and then add it to the bowl and beat in to the mixture. Again, it may look like there is too much egg at first but it will come together.
Take the pastry circles out of the fridge. Spoon the filling onto the smaller circle of pastry, leaving about 2cm clear around the edges. Hide the charm somewhere in the filling, around the edge is best so that you have less chance of hitting it with your knife when you are slicing the Galette.
Beat the other egg, and brush around the edge of the disc of pastry. Place the larger disc over the top and press firmly around the sides to join the two layers together. Trim the excess pastry from the larger disc so that it looks neat.
Brush egg over the Galette, trying to make sure that the egg doesn’t drip down the side of the edges, as this may prevent it from rising properly.
Use a sharp knife to make a small hole in the top, and then to mark lines down the sides like the spokes of a wheel.
Bake for 25-30 minutes until the Galette is golden and puffy. Serve warm or cold.