Preheat oven to 375°F (190°C).
Lightly grease a small 5 × 7-inch baking dish or two oven-safe ramekins.
Peel, core, and slice the apples approximately ¼-inch thick.
Toss apples with lemon juice.
Add brown sugar, cinnamon, vanilla, and cornstarch.
Toss until evenly coated.
Transfer apples to the prepared baking dish.
Combine oats, flour, brown sugar, cinnamon, and salt in a small bowl.
Add cold butter pieces.
Work butter into the dry ingredients with a fork or fingertips until irregular crumbs form.
Scatter the topping loosely over the apples.
Bake for approximately 28–35 minutes.
Check that the topping is golden and the apple juices are bubbling.
Test an apple slice for tenderness.
Remove from oven.
Rest for 10–15 minutes.
Divide between two servings.
Serve warm, optionally with ice cream.