Preheat oven to 375°F (190°C). Grease a 9×13-inch (23×33 cm) baking dish.
Prepare the filling: Toss apples with lemon juice (and zest, if using). Add brown sugar, granulated sugar, cinnamon, nutmeg, vanilla, and cornstarch (or flour). Spread evenly in the pan and dot with butter.
6-7 cups peeled and chopped apples, 2/3 cup brown sugar, 1/3 cup granulated sugar, 1 1/2 tsp cinnamon, 1/2 tsp nutmeg, 1 1/2 tsp vanilla extract, 3 tbsp 24 g cornstarch (or 6 tbsp / 48 g flour if substituting), 3 tbsp unsalted butter, cut into small pieces, 1-2 tbsp fresh lemon juice, 1 tsp lemon zest
Prepare the topping: Whisk together flour, sugar, baking powder, salt, and cinnamon. Cut in cold butter until crumbly.
1 1/2 cups all-purpose flour, 1/3 cup sugar, 2 tsp baking powder, 1/2 tsp salt, 3/4 tsp cinnamon, 1/3 cup cold unsalted butter, cubed
Activate starter: In a small bowl, mix sourdough starter with baking soda. Let sit 1–2 minutes. Stir into dry ingredients along with milk and vanilla until just combined.
2/3 cup sourdough starter or discard, 1/2 tsp baking soda, 1/3 cup milk, 1 1/2 tsp vanilla extract
Assemble: Drop spoonfuls of topping over apples. Some filling peeking through is fine.
Bake: 30–35 minutes, until topping is golden and apples are bubbling.
Serve: Serve on its own or with vanilla ice cream.