In the morning, feed your starter. You will need an active starter that's doubled in size after feeding. I mix 40g of starter with 40ml of water and 40g of flour and leave at 21-25°C for 4-5 hours to double in size.
When your starter reaches its peak, in a bowl of your standing mixer, combine starter, milk and half of the flour. Cover and set aside for 1 hour.
Lightly whisk a large egg in a small bowl. Take out about 2 tablespoon of it and reserve for later. Add the sugar, melted butter, salt and the beaten egg. Using a dough hook attachment, knead on low speed, adding 1 tablespoon of the remaining flour at a time. Once all flour is incorporated, knead for another 3 minutes.
In the meantime, lightly grease a shallow dish. Tip the dough into the bowl and turn once to cover the dough in oil. Cover with a teatowel.
Over the next 3 hours, perform a set of coil folds (or stretch and folds) every 30 minutes (see more details regarding the methods in the text above). After 6 sets of folds, leave the dough to rise for another hour. It should be lively and elastic by now.
Generously grease an ovenproof dish with butter (I used an 18cm (7in) cake tin, to get more of a tear & share bread effect, but you can use a bigger tin for the buns that are just touching).
Prepare your filling. Peel, core and chop a large apple into small cubes. Squeeze half a lemon over the apple cubes and mix to coat. In a small bowl, mix cinnamon and brown sugar.
Scoop the dough out onto a well-floured surface taking care not to knock too much air out. With a rolling pin, roll the dough out into a 40x25cm (16x10in) rectangle. Brush the surface with the reserved egg.
Sprinkle the dough with cinnamon and sugar mixture and use your hands to press it in, so that it sticks to the surface. Then spread the apple cubes all over.
Working from the short side, tightly roll the dough into a log. Use a sharp knife to cut it into 8 equal rounds. Arrange them snuggly (or not, depending on the tin you're using) in the ovenproof dish. If you use a smaller tin, they will rise upwards, if you use a large one, they will spread sideways and slightly upwards.TIP: if you're using a springform cake tin, you may want to thightly wrap some foil around the bottom of your tin. Apples will release juices and sugar will sissolve creating a syrup. Foil ensures the syrup cannot escape and make a mess in your fridge or oven.
Brush the top with 1 tablespoon of melted butter, cover with clingfilm and pop in the fridge to cold-prove overnight.