Place the apples into a large bowl and toss with the cornstarch, 1 teaspoon of the cinnamon, and the lemon juice. Spread into the prepared baking dish.
Place the oats, flour, sugar, vanilla, and remaining 1 teaspoon cinnamon into a medium bowl.
Use your fingers to work the butter into the mixture to until uniformly mixed. There can be some chunks of butter remaining. The texture should be similar to a chunky granola. (Alternatively, you can do this step in a food processor.)
Spread the topping over the apples in the pan. Bake for 30-35 minutes or until the apples are soft. If the topping starts to get too dark brown at any point, cover it loosely with a piece of foil and continue baking. Let cool slightly and serve warm or at room temperature.
You can make this up to 3 days ahead and store it in the fridge or up to 3 months and store it, lid on or otherwise tightly covered, in the freezer. Thaw overnight in the fridge and warm through in a 300 degree oven, covered with foil.
I like to use Cortland, Granny Smith, or any other cooking apple in this recipe.
Serve with whipped cream or vanilla ice cream as desired.
Serve leftovers for breakfast with plain Greek yogurt.