Lightly grease the sides and bottom of a 9"x 13" baking dish with butter and arrange the sliced bread in a single layer.
In a large bowl, whisk together the eggs, milk, salt, sugar, vanilla and ground spices until combined well.
Pour this mixture evenly over the bread slices.
In another bowl, toss the apples with the sugar, butter, lemon juice and powdered cinnamon. Arrange the apple slices over the bread in a single or 2 layers.
Cover the dish with clingfilm and refrigerate overnight or for up to 2 days.
Preheat the oven to 190 degrees C. Remove the baking dish from the fridge and bake the French toast for 45-50 minutes or till the egg custard has set. Serve warm.