Unroll the pastry sheet straight onto a baking tray and cut in half – like a book.
Slice the brie through the middle to make two rounds, and pop the bottom half in the middle of one of the pastry halves. Spread the chutney over it then sprinkle on the red onion and herbs. Sandwich the top of the brie back on and squish down.
Brush the pastry around the cheese with some water, then flip on the second pastry half. Press it all the way around the cheese to stick, then trim the excess leaving a 2-3 cm pastry border. Seal by pressing with the prongs of a fork, then brush the whole lot with egg yolk. Chill for 30 minutes (at least) and preheat the oven to 220°C (200°C fan oven) Gas Mark 7.
Bake the pastry-wrapped cheese on the top shelf for 18-22 minutes until golden and puffed. Enjoy immediately, with extra bread or breadsticks for dunking and scooping.