Start by cutting the puff pastry into 6 equal-sized pieces and set aside.
Into a bowl, add the ground beef, the onion, the salt, the black pepper and the water. Mix very well by hand until combined.
Get a chunk of meat and round it into a little ball. I've made 5 balls in total.
Cut the mozzarella cheese into 6 dices.
Place each meatball in the center of each square of puff pastry, then place each cheese dice on top of each meatball, gently squeezing it down into the meat.
Grab the tips of each square of pastry and fold them towards the center, forming a little ball - make sure it's totally sealed.
Repeat this process with every square of pastry.
Transfer the balls to a baking sheet lined with parchment paper and brush the mixture of egg yolk and table cream over each of them.
The cut start-shaped slits on the surface of each ball using scissors.
Open the slits.
Add another mozzarella dice, about 50g, into each of these holes.
Bake in a preheated oven at 180°C (356°F) for 40 to 60 minutes.
Serve.