The night before you plan to make this pancake, mix your sourdough starter with the flour and milk, then let it sit overnight at room temperature, covered with a cloth.
The morning of, stir your fermented flour and milk mixture well. Add the eggs, baking soda, brown sugar, cinnamon, and salt. Mix well.
Place a cast-iron skillet inside the oven and heat to 450°F.
When the oven is hot, remove the skillet, add 1 tablespoon butter, and swirl until melted. Sprinkle in a very thin layer of your favorite sugar, then add your fruit to the bottom of the pan. Add the pancake batter.
Return the pan to the oven for 15 to 20 minutes. The pancake is ready when the top is evenly browned and the edges are crisp.
Flip your sourdough pancake onto a plate, fruit side up, and serve hot with maple syrup and butter on top.