Prepare your baking pan by lining a 9×13 inch baking dish with parchment paper, leaving some overhang on the sides for easy removal later. Preheat your oven to 350°F (175°C).
Make the crust by combining flour, sugar, and salt in a large bowl. Cut in the cold vegan butter using a pastry cutter or fork until the mixture resembles coarse crumbs. Add the cold water and mix until the dough just comes together.
Press the dough evenly into the bottom of your prepared pan. Use the bottom of a measuring cup to create a smooth, even surface. Prick the crust all over with a fork and bake for 15 minutes until just beginning to turn golden.
Prepare the apple filling while the crust is baking. Combine the sliced apples, brown sugar, lemon juice, cornstarch, and spices in a large bowl, tossing gently until all slices are evenly coated.
Make the crumble topping (if using) by combining oats, flour, brown sugar, salt, and cinnamon in a bowl. Pour in melted vegan butter and stir until everything is moistened and clumps form.
Arrange the spiced apple slices in an even layer on the par-baked crust, slightly overlapping them for even baking. Pour any remaining juices from the bowl over the apples.
Sprinkle the crumble topping evenly over the apples if using. If not using the crumble, consider sprinkling the top with a little extra cinnamon sugar mixture.
Bake for 30-35 minutes until the filling is bubbling and the top is golden brown. If the edges start browning too quickly, cover them loosely with foil.
Allow the bars to cool completely in the pan for at least 2 hours before lifting out using the parchment paper overhang and cutting into bars. This cooling time is essential for the filling to set properly.