Grease an ovenproof casserole dish well. Caramelise 100 g sugar in a pan until golden. Pour the caramel into the mould immediately and allow to cool.
Cut the crust off all around the bread. Cut the bread into about 12 slices and place them in about 3 layers in the mould. Cut the vanilla pod lengthwise and scrape out the pulp. Wash the lemon hot, dry it and finely grate the peel.
Warm the milk lukewarm. Stir in 200 g sugar, vanilla pulp and lemon peel. Leave to cool slightly. Cream vanilla milk and eggs with the whisk of the mixer. Pour evenly over the slices of bread and leave to stand for about 10 minutes.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Peel, quarter, core and cut apples into fine slices. Spread on the bread. Mix 1 tbsp. sugar and cinnamon and sprinkle over the bread.
Bake the casserole in a hot oven for about 40 minutes. Remove, dust with icing sugar and serve. Chocolate sauce tastes good with it.