Heat cherry jam and 100ml water in a pan over low heat until melted.
Stir in the arrowroot paste and simmer until the sauce is thick.
Coat cherries in the jam mixture and transfer to a 1 litre pie dish to cool.
Pulse flour, butter, and 1 tablespoon sugar in a food processor into fine breadcrumbs.
Gradually add the beaten egg and 1 tablespoon water mixture to form a dough.
Roll pastry on a floured surface to 7cm larger than the dish and cut edge strips.
Brush the dish rim with egg and layer the pastry strips around it.
Place the pastry lid over the filling, seal the edges, trim, and crimp.
Place the pie on a tray and chill in the fridge for 30 minutes.
Preheat the oven to 200°C.
Brush the lid with egg, sprinkle with 2 teaspoons sugar, and cut a small cross in the center.
Bake at 200°C for 25 minutes, then at 180°C for 15 minutes until golden.
Serve the pie hot.