Preheat oven to 350°F (175°C) and grease a 9x13-inch dish.
Toss apples with lemon juice, then add spices & cornstarch.
Combine oats, flour, sugar, baking powder, salt & cinnamon.
Whisk butter, milk, egg, and vanilla in a separate bowl.
Stir wet into dry until just combined.
Spread apples evenly in baking dish.
Crumble oat mixture over apples in an even layer.
Bake for 45–50 minutes until golden and bubbly.
Cool for 10–15 minutes before serving.