Creamy Chicken Puff Pie

x 320g/11oz rolls of ready-rolled puff pastry

chicken breasts (cut into small chunks)

carrots (peeled and chopped into chunks)

medium sized potatoes (peeled and chopped into small chunks)

good pinch of salt and pepper

sprigs of thyme or 1 tsp dried thyme

chicken stock (water plus 3 stock cubes is fine)

butter

large onion (peeled and finely chopped)

plain/all-purpose flour

milk (half or full fat)

Juice of 1 lemon

egg (slightly whisked with a fork)