Melt the butter in a saucepan, add the pieces of apple, blueberries, lemon zest and juice and sugar, and then stew until the apples are more or less soft. Dissolve the corn starch in a little bit of cold water and add to the mixture. Heat up once more and then set aside to cool down. Place slices of bread at the bottom of an oven dish, spread the apple-blueberry filling on top and add another layer of bread. Mix up the eggs, sugar, cinnamon and vanilla, pour the mixture evenly into the dish. If you wish, sprinkle with sugar and cinnamon. Cook at 180 °C until golden brown.