Cook the beetroot until done. This takes approx. 1 hour. You should be able to poke the beetroots with a knife.
Allow to cool and peel. Cut into equally-sized cubes with as little waste as possible. Drill a hole in the middle of each piece, 1 cm in diameter, with an apple drill. Stuff this hole with soft goat cheese.
Flange
Place the flour and cornstarch in a bowl. Add milk to this while whisking. Then, add the egg, and finally the herbs. Place aside for 15 minutes.
Then, bake them in an elongated shape on both sides. Place aside to cool.
Finishing
Place the stuffed beetroots next to each other with the hole in the same direction.
Roll out the puff pastry until it has the appropriate size to cover the beetroots.
Wrap the flange around the beetroots and place it all on the puff pastry. Close it by carefully wrapping the puff pastry around the flange. Brush the sides with egg yolk and close. Cut away any surplus puff pastry. You could use this for decoration. Brush the outside with egg yolk and bake in the oven at 180ºC, until the pastry is golden brown. This takes approx. 20 minutes. Cut open when it is still hot.