3In a mixing bowl, combine 0.75 cups of butter, 0.5 cups of light brown sugar, 2 cups of flour, and a pinch of salt.
4Mix until small lumps form.
5Press the mixture into the prepared baking dish.
6Bake at 300°F (150°C) for 15 minutes.
7Meanwhile, prepare the streusel topping: In a mixing bowl, combine 0.75 cups of flour, 0.75 cups of oats, 0.75 cups of light brown sugar, 0.75 tsp of baking powder, a pinch of salt, and 0.5 tsp of cinnamon.
8Add 0.5 cups of cubed butter and mix with a fork.
9Refrigerate the streusel topping.
10Prepare the apple filling: Peel and slice 4 large Granny Smith apples.
11In a bowl, combine the sliced apples, 1 tbsp of lemon juice, 2 tbsp of all-purpose flour, 2 tbsp of white sugar, 1 tsp of cinnamon, and a splash of vanilla.
12Mix until apples are fully coated.
13Remove the base from the oven, increase the temperature to 350°F (175°C).
14Add half of the refrigerated streusel topping to the bottom of the baking dish.