Preheat oven to 375°F. Grease a 9-inch pie dish.
Toss apples with lemon juice, 0.5 cup sugar, 1.5 tsp cinnamon, nutmeg, cornstarch, and 0.25 tsp salt.
Spread apples evenly in the prepared dish, mounding slightly in the center.
Whisk flour, 0.33 cup sugar, baking powder, and 0.25 tsp salt in a bowl.
Stir in milk and vanilla until a smooth batter forms. Do not overmix.
Pour batter evenly over the apples, leaving a few gaps around the edges.
Drizzle melted butter over the batter to help it crisp and brown.
Combine 1 tbsp sugar with 0.25 tsp cinnamon. Sprinkle over the top.
Bake 45–50 minutes until the crust is deeply golden and the apples are tender.
Cool 20 minutes before slicing. Serve warm or at room temperature.