Slice and pit the peaches, cutting them into chunks. You do not need to peel the peaches.
In a large bowl, combine the sliced peaches with the remaining filling ingredients (brown sugar through lemon juice). Mix until combined. Transfer the peach mixture into a 9x13-inch baking pan (or a 3-4 quart casserole dish) and spread evenly. Bake in the preheated oven for 10 minutes.
While the peaches bake, prepare the topping. In a medium bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt. Add the cold, cubed butter, sourdough discard, and 1 Tablespoon of milk. Use a pastry cutter or fork to mix until you have a rough dough. The dough should hold together and might be slightly sticky, and you should still see butter chunks. If needed, add a little more milk to reach the desired dough consistency.
Remove the peaches from the oven. Using a spoon, drop spoonfuls of the topping mixture on top of the peaches, leaving room between each spoonful.
Brush the topping with egg wash, then sprinkle with turbinado sugar. Bake for 30-35 minutes until the filling is bubbling and the topping is golden brown.
Let the cobbler cool slightly, then serve topped with a scoop of vanilla ice cream.
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Notes
Using frozen peaches - If you're using frozen peaches, let them thaw and drain off excess liquid before adding to the recipe. Frozen peaches are a great alternative to using fresh fruit.A note on canned peaches - I don't recommend making this recipe with canned peaches. They are cooked during the canning process, so they tend to get too mushy when you bake them into a cobbler. They also have a lot more sugar and can make the cobbler overly sweet.To make this recipe with active sourdough starter, replace the sourdough discard with active starter (200g). You do not need to change any other ingredients in the recipe.