Preheat your oven to 350°F (175°C). This ensures that the cobbler cooks evenly and the topping becomes golden brown.

In a large mixing bowl, combine the sliced peaches, granulated sugar, lemon juice, vanilla extract, cornstarch, and cinnamon (if using). Toss gently to coat the peaches evenly. The sugar will draw out the juices from the peaches, creating a delicious syrup.

Transfer the peach mixture to a greased 9x13 inch baking dish, spreading it out evenly.

In a separate mixing bowl, whisk together the sourdough starter, all-purpose flour, milk, melted butter, granulated sugar, baking powder, salt, and beaten egg until just combined. Be careful not to overmix; a few lumps are okay.

Using a spatula, dollop the sourdough mixture over the peach filling. Spread it out gently to cover as much of the peaches as possible, but it’s okay if some peaches peek through.

Place the baking dish in the preheated oven and bake for 40-45 minutes, or until the topping is golden brown and a toothpick inserted into the topping comes out clean. The bubbling peach filling will be visible around the edges.

Once baked, remove from the oven and let it cool for about 10-15 minutes before serving. This allows the filling to set slightly, making it easier to serve.

Serve warm, optionally topped with vanilla ice cream or whipped cream for an extra treat. Enjoy your delicious sourdough peach cobbler!