In a saute pan over medium heat, add butter. Once melted, add apples, brown sugar, salt and cinnamon. Gently saute, then reduce heat and let simmer until batter is ready to bake.
In a mixing bowl combine sugars, oil, eggs, and sourdough. Combine flour, cinnamon, salt and baking powder then add to mixing bowl. Thoroughly combine with wet ingredients.
Line the baking dish with parchment paper. Pour batter into baking dish and shake out to settle evenly. Pour apple mixture over the top, evenly.
Place in oven and bake for 50 minutes. Then check with toothpick to come out clean. Bake in 5 minutes increments until cooked through.
Remove from oven but allow to cool in the baking dish for 30 minutes. Then remove from dish (using parchment paper). Slice and serve still slightly warm.