Potato gratin with rosemary

1 pack of puff pastry
6 floury potatoes
200 ml milk
1 egg
chopped rosemary
chopped green onion
salt and freshly ground white pepper
Roll out the chilled puff pastry into a thin sheet between two pieces of SAGA Cooking Paper. Line a casserole dish with a sheet of cooking paper and place the puff pastry into the dish.
Peel and thinly slice the potatoes and place on top of the puff pastry.
Pour the milk into a saucepan, add the rosemary and bring to a boil. Pour the milk through a sieve to remove the rosemary. Beat the egg in with the milk and pour over the potatoes. Sprinkle the chopped green onion, salt and freshly ground white pepper on top.
Bake in an oven for about 40 minutes at 180°C (200°C).