Preheat the oven to 180 degrees C. Lightly coat a rectangular ovenproof dish with cooking spray.
Fruit Layer: Put the brown sugar, flour and cinnamon in a food processor. Pulse to combine. Alternatively, use your fingers to mix the ingredients together in a medium bowl.
Pour the lemon juice into a large bowl. Peel the apples and cut into 5mm slices. Add to the bowl, tossing with the juice. Sprinkle with the brown sugar mixture and add the vanilla essence. Toss until evenly coated. Turn into the prepared ovenproof dish.
Topping: Combine the oats, flour, brown sugar and cinnamon in a medium bowl. Using your fingers rub the spread into the flour mixture until it resembles coarse crumbs. Sprinkle over the apples. Bake for about 40 minutes or until the topping is golden and the apples are tender.