Preheat oven to 340 F.

To make custard: Place custard powder and 2 tablespoons sugar in jug with milk then stir until fully dissolved. Add grapefruit juice and stir.

Transfer mixture to microwave and heat 3 minutes, stopping every minute to stir, until custard is thick and smooth.

Roll out puff pastry and use sharp knife to score 1-centimeter border around edge.

Transfer pastry sheet to nonstick baking tray lined with parchment paper. Brush edges with beaten egg and sprinkle with 1/2 tablespoon sugar for crispy, caramelized finish.

To assemble galette: Spread custard evenly over pastry, staying within border. Layer sliced apples on top to fully cover custard. Finish by lightly sprinkling remaining sugar over apples.

Bake 25 minutes, or until pastry is golden and crisp.

Just before serving, zest fresh grapefruit over galette.