After 5 minutes, sprinkle the top with caster sugar and change your oven setting to "grill" (broiler) to brown the top (if your grill is separate, leave the dish in the oven for a couple of minutes longer, then brown briefly under the grill).
Remarks
Use eating apples, rather than tart cooking apples for this, as the lemon in the confectioner's custard already adds a note of acidity.
You will notice that the cooking time is very short - just 10 minutes - 5 minutes of which are under the grill. The clafoutis only really needs to be heated through and the top browned.