Preheat the oven to 350°F (175°C) and line a 9x9-inch baking pan with parchment paper.
Prepare the shortbread crust. Place the flour, powdered sugar, and salt in a food processor. Add cubed cold butter, and pulse until a crumbly texture forms.
Transfer the shortbread to the prepared baking pan. Press the dough into the pan, and par bake for 20 minutes.
In a separate bowl, toss the apple slices with maple syrup, lemon juice, cinnamon, nutmeg, and cornstarch. Make sure the apples are evenly coated.
Prepare the streusel topping. Combine the rolled oats, all-purpose flour, brown sugar, and cinnamon in a bowl. Whisk to combine. Add the vegan butter. Use a fork or pastry cutter to cut the butter into the dry ingredients until a crumbly texture forms.
Spread the apple filling evenly over the shortbread crust in the baking pan.
Spread the streusel topping evenly over the apples.
Bake for 30–40 minutes until the top is golden brown and the apples are tender. The edges should be slightly crisp.