Preheat the oven to 400F/200C and line a baking sheet/tray with parchment.
Open up the sheet of puff pastry and smooth out any folds. If needed, roll out slightly so that it will be large enough to wrap around the brie.
Place the brie in the middle of the pastry and lightly score the top crust of the cheese. Spread roughly half, or a little more, of the honey over the top of the cheese to form a layer on top.
Roughly chop the cherries and the pistachios and mix them together. Stack the mixture on top of the honey-topped brie - it may well be a bit full, but it tends to compact a little as it cooks so try to fit it all on. Drizzle the rest of the honey over the top which helps it hold together a little.
Fold in the pastry around the brie and the toppings and seal it over the top. Try your best to fold relatively evenly so you neither have gaps (particularly around the sides) nor areas that are very thick.
Lightly beat the egg and use some to help seal the pastry together, getting under folds. Stretch pastry as needed to help seal. Transfer the wrapped cheese to the lined baking sheet then brush all over with egg wash.
Bake the cheese in the oven for approximately 20 - 25 minutes until the pastry is golden brown and slightly puffed up all over. Allow to cool for a good 5 - 10 minutes before cutting so the cheese is hopefully a little less liquid, but bear in mind it will run so place on something that will catch the drips. Serve with crackers and apple slices.
Video
Notes
See notes above about alternative toppings that you can use.