Preheat the oven to 180C/350F/Gas 4.
Cut the apples in half horizontally, remove the cores and slice off the rounded tops and bottoms of the apples so they sit flat in the tin.
Put the water in the bottom of the tin and sprinkle over the sugar. Let it sit for two minutes to allow the water to absorb the sugar.
On a medium heat, cook the syrup until it turns to a pale blonde caramel (do not let it become too dark or the flavour will overpower the apples), then stir in the diced butter - this will stop the caramel cooking and give extra richness to the dish.
Arrange the apples, middles uppermost, around the edge of the tin and then fill in the centre with the remaining apples. It is important to pack them as tightly as possible – press them down with your hands as you go.
Brush the apples with the melted butter and place the tin on a baking tray. Bake in the oven for 30 minutes.
Remove from the oven and place the disc of frozen pastry on the top. Tuck the edges of the pastry into the dish and prick a few holes with a sharp knife to let the steam out as the dish cooks.
Return to the oven for a further 40-45 minutes, or until the pastry is crisp and golden-brown.
Allow to cool at room temperature for one hour before turning out of the tin and serving warm. Alternatively, make the tarte the day before and reheat as above.