Mix the sourdough starter with the flour and milk. Let it sit at room temperature loosely covered with a cloth, overnight and up to 24 hours.
After the starter, milk, and flour have sat, stir it down and wisk in the eggs, baking soda, salt, and sugar. The batter should be airy and like a milkshake. It it seems too stiff, add just a little milk.
Preheat the oven to 450 degrees F, with your cast iron skillet inside the oven. Wait 5-10 minutes after the oven comes up to temperature to let the skillet get good and hot. Take the skillet out of the oven (use oven mitts) and swirl in the butter until it melts.
Add your fruit in a thin layer so that you can still see some parts of the bottom of the pan. If using firm fruits like apples or pears, put the pan on medium heat and let the fruit sizzle a little until it softens a little. If using softer fruits or berries just add the batter right away. Add enough batter to totally cover the fruit and make a pancake about 1/2 inch in thickness
Return the pan to the oven for 15-18 minutes until the top is evenly brown and the edges are crisp. Flip the pancake onto a plate with the fruit side up and serve hot with more butter and a good splash of syrup.