Preheat your oven to 325 degrees.
Peel, core, and slice your apples and place in a 9”x13” baking dish.
In a small bowl, mix together the granulated sugar, 2T flour, and cinnamon. After you have mixed this together, pour over the prepared apples in the baking dish.
Next, take the water and pour over the top of the apples and sugar mixture.
In a medium bowl, add the oatmeal, brown sugar, baking soda and cinnamon. Mix to combine. Then in a separate small bowl combine the melted butter and sourdough starter (or substitute flour for the sourdough starter and add it to the bowl with the oatmeal). Mix well and then add to the bowl with the dry ingredients.
Once well combined, spread the oatmeal topping over the apples in an even layer.
Place in preheated oven and bake for 60-80 minutes until you have tender apples (crisp apples will take longer to bake), the juices are bubbling, and the top is nicely golden brown.
Remove from the oven. Serve with ice cream or whipped cream.
Store leftovers at room temperature 2-3 days in an airtight container or covered with plastic wrap.