Preheat oven to 180°C bake.
Peel apple. Cut into quarters, remove core and slice into 1cm-thick slices.
Heat an ovenproof frypan on medium heat. Add sugar and butter to pan and stir until melted. Add apple slices and toss to coat. Remove from heat and carefully arrange apple slices into a single layer.
Cut pastry in a circle slightly larger than pan. Poke holes into pastry using a fork. Carefully lay pastry over the top of the apples, tucking it in around the edges using a spatula or butter knife. Bake for 35-40 minutes, until pastry is puffed and golden.
Leave to rest for 5 minutes, then turn out onto a plate or board, so the apple side faces up.
If desired, heat custard in a small pot on low heat for about 2 minutes or in a small bowl in the microwave for 30 seconds at a time, until warm.
Serve slices of tart with a dollop of custard.