Grease your baking dish (or cast-iron skillet) with a knob of butter, and then dust it with 2-3 tablespoons of fresh breadcrumbs (if using). Set aside for now.
Prepare the fruit
In a bowl, soak 100g (3½ oz) of sultanas or raisins in 120ml (½ cup) of hot strong black tea, and set aside to plump up.
Peel and core 500g of apples, then cut into slices. I prefer mine to be slightly on the thicker side - c. 5mm (¼ inch). Feel free to cut yours thinner if you like.
Place the apple slices into a large mixing bowl, and cover with cold water and 1 tablespoon of lemon juice (this will help to stop them going brown).
Mix together 2 tablespoons of granulated white sugar and 1 teaspoon of ground cinnamon. Set this aside for now.
Make the custard
In another large mixing bowl, whisk together 400ml (1⅔ cups) of whole milk, 120ml (½ cup) double / heavy cream, 3 medium eggs, 100g (½ cup) granulated white sugar, and ½ teaspoon of vanilla extract.
If you're not going to rest the assembled žemlovka before baking, pre-heat your oven now to 180° C (350° F, gas mark 4).
Prepare the bread
Slice the bread; I like mine to be quite thick - around 1,5cm (½ inch).
Assembly
Dip several slices of bread into the custard, and arrange them in a single layer on the bottom of your prepared pan.
Drain the sultanas / raisins (keep the rum, if used - it'll taste lovely; otherwise, discard the cold tea), then spread them over the bread.
Drain the apples, and blot dry with a clean tea towel.
Next arrange the sliced apples on top of the sultanas (or raisins) in an even layer, and sprinkle over the cinnamon and sugar mix you made earlier.
Dip the remaining slices of bread, and arrange on top of the apples. Drizzle any remaining custard mixture over the top, and gently press down on the bread with a fork or flexible spatula in order to soak up as much as the custard as possible.
Cut up 75g (c. 5 tablespoons) of cold butter, and dot pieces over the top of the žemlovka.
Rest (optional)
Cover the žemlovka with aluminium foil (or a stretchy silicone lid), and place in the fridge for 1 hour.
Bake
Place in the centre of the oven, and bake for around 40 minutes, until the top is golden brown and crunchy.
To serve
Remove from the oven, and set aside for 15 minutes. Just before serving, sprinkle with 2 tablespoons of icing sugar (powdered sugar).
Storing leftovers
Fridge - in an airtight container for up to 3 days.
Freezer - this recipe is not suitable for freezing.
Reheating
Cut into thin slices, or break into chunks, and fry in hot butter until crispy.
Notes
Nutritional data has been calculated on the assumption that brioche and salted butter are used.