Preheat oven to 180c. Grease an 8 cup baking dish. Tear each croissant into thirds and roughly scatter half over the base of the dish. Top with apple and sultanas and finish with remaining croissants.
In a large jug, whisk eggs and milk together until smooth. Add custard and whisk again until combined. Pour over croissant mixture, pushing croissants down into the liquid. Set aside for 15 minutes.
Scatter almonds and sugar over top of pudding and bake for 40 minutes until golden and custard is just set. spoon into serving bowl while warm and serve.