Preheat the oven to 350°F (180°C). Butter and line a 9x13-inch (23x33 cm) baking dish with parchment paper. Set aside.
Make the Streusel Topping: In a medium bowl, combine the flour, sugar, oats, salt, and cinnamon.
Rub in the cold butter with your fingers (or use a pastry blender) until the mixture is crumbly, then toss in the pecans. Refrigerate until ready to use.
Make the Apple Layer: In a medium bowl, combine the chopped apples with the flour, brown sugar, cinnamon, melted butter, and lemon juice. Set aside.
Make the Blondie Base: In a medium bowl, whisk together the brown sugar and melted butter, then whisk in the eggs and vanilla extract.
In a small bowl, mix the flour, baking powder, and salt, then stir into the brown sugar mixture until just combined.
Spread the blondie base evenly into your prepared pan, then top with the apples and finally the streusel topping.
Bake for 45 - 55 minutes, until the apples are tender and the topping is nicely browned. Let cool completely in the pan before slicing and serving.
This is best the day it is made but store leftovers in an airtight container in the refrigerator for up to 2 days. Serve with Vanilla Ice Cream and Apple Cider Caramel Sauce on top.