Preheat the grill to hot and prepare the French toast batter by whisking together the eggs, caster sugar, milk and salt.
Melt half the butter in a large, wide ovenproof frying pan set over a medium heat and add the apple slices. Sprinkle the light soft brown sugar on top and toss to coat well. Cook, tossing occasionally for 5-6 minutes.
Meanwhile, dip the bread in the batter and melt the remaining butter in a separate frying pan over a moderate heat. Fry the bread in batches until golden on the outside.
Layer on top of the apples and transfer to the oven. Grill for a few minutes until golden on the outside then remove and serve from the pan.
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