Preheat the oven to 175°C (350°F). Grease and flour the pan, or line the entire pan with parchment paper.
Note: 4x servings (~12 pcs) fit in a 22x33 cm or 9x13-inch
In a medium bowl, sift together flour, baking soda, cinnamon, and salt. Set aside.
Combine the peeled, cored, and sliced apples with white sugar in a bowl. Set aside to allow the apples to release their juices.
In a large mixing bowl, whisk together brown sugar, vegetable oil, eggs, and vanilla extract until smooth.
Stir the apple mixture into the wet ingredients. Gradually mix in the dry ingredients until just combined. Fold in the chopped walnuts.
In a small bowl, mix brown sugar, flour, cinnamon, and butter. Break it into small crumbs using your hands or a fork.
Pour the batter into the prepared pan. Sprinkle the crisp topping evenly over the top. Bake for 45 minutes, or until a toothpick inserted in the center comes out clean.
Let the cake cool completely in the pan before slicing. Serve plain, with whipped cream, or a drizzle of caramel sauce.
