Preheat the oven to 375°F (190°C). Lightly butter a 9X13-inch baking dish, or a cast iron pan. In a large mixing bowl toss the sliced apples, brown sugar, salt, vanilla extract, allspice, water (or apple juice), cinnamon, nutmeg, lemon juice, butter, and flour.
Place the apples in a cast-iron pan or skillet and cook on medium heat for 3-5 minutes until apples are slightly soft. Apples should be slightly tender but not super soft!
While the apples are cooking, in another large bowl, whisk together the flour, baking powder, baking soda, salt, oats, and sugar.
Grate or cut in the cold butter using a pastry cutter, fork, or your fingers until the mixture looks like coarse crumbs.
In a small bowl whisk together the sourdough starter, buttermilk, and vanilla.
Add the wet ingredients to the dry ingredients and stir just until combined. It should be a thick, slightly sticky dough.
Scoop dollops of the cobbler dough evenly over the slightly cooked apples. It doesn’t have to cover them completely--gaps are fine!
Bake in a preheated oven for 35–45 minutes, or until the topping is golden brown and the apple filling is bubbling around the edges.
Serve warm, optionally with a scoop of vanilla ice cream (or your favorite ice cream) or a dollop of homemade whipped cream.