Into a bowl, add the strawberries, the blackberries, the raspberries, the sugar and the juice squeezed from the lemon, mix all the ingredients very well and let it rest for 20 minutes.
After this resting time, transfer all the fruits to a large saucepan, turn on the heat to low temperature, stir well and bring to a boil. When that happens, stir again then stir every 5 minutes until the syrup is well reduced.
Now with a premade pie crust, cover the bottom and the edges of a pie mold.
Take the pie crust to a preheated oven at 230°C (446°F) and let it bake for 5-10 minutes.
Now, get another piece of pie crust and cut it in the shape of a very spooky ghost face. Make holes for the eyes, nose and mouth. You can cut little bits of crust to use as teeth.
Once the crust has baked through, remove it from the oven and fill it with all the berry jam.
Cover the jam with the ghost-face crust, removing the excess and shaping it until well-finished.
Take it back to the preheated oven and let it bake at 200°C (392°F) for another 20 minutes.
Then let it cool for a while and take it to the fridge to chill.
Serve.