In a large mixing bowl weigh out the starter, milk, brown sugar, and butter. Mix until combined then add the eggs and mix well. Next, add the flour to the large bowl and mix well making sure that the mixture is well incorporated.
Cover the bowl and let ferment in the fridge overnight or 10-12 hours.
The next day in a medium sized mixing bowl add the apple, baking soda, cinnamon, salt, nutmeg and sugar. Mix until the apples are well coated and then add to the dough mixture.
Using your hands fold and incorporate the apple mixture into the dough. Let rest for 1 hour.
About 30 minutes before you are ready to fry the apple fritters take a large pan with a heavy bottom like a Dutch oven or tall sided cast-iron skillet. Add 1.5 inches of oil in the bottom and using a thermometer, such as a candy thermometer, heat the oil to 350-375 degrees.
You want to make sure to keep the oil in this range for the best outcome on the fritters. You will most likely need to turn the heat up when you first put the apple fritters in the pan so that they can maintain the 350–375-degree oil temperature. I usually fluctuate the heat on the stove from a medium heat to a low heat.
While you are waiting for the oil to preheat take a small bowl and add the powdered sugar and ¼- ½ cup of milk. Add the milk whisking until you get a thin consistency.
Take the sourdough apple fritter batter after it has sat for 1 hour at room temperature and taking roughly a ¼ cup of batter stretch it out with your hand to get it flat and then carefully set the dough in the hot oil. I usually only do 2 at a time but you may be able to do 3 at a time depending on the size of the pan and if you are able to watch the temperature closely.
You will want to do roughly 3-4 minutes on each side. You want them to be golden brown and cooked through. Then when both sides are cooked transfer the apple fritters using a slotted spoon or slotted spatula to a plate lined with a paper towels. This helps get rid of any excess oil.
Place more apple fritters in the batter and then transfer the ones you have on the plate to a cooling rack. Dip these in the glaze on each side and then transfer back to the cooling rack to cool and dry.
Repeat with the remaining batter and enjoy!
Serve warm or same day for best results. Store any leftovers in an airtight container.