Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan.

In a large mixing bowl, combine the flour, baking soda, cinnamon, and salt. Whisk together until well mixed.

In another bowl, beat together the sugar, eggs, and vegetable oil until smooth.

Stir in the applesauce into the egg mixture until combined.

Gradually add the dry ingredients to the wet ingredients, stirring until just combined.

In a small bowl, mix together the chopped walnuts, brown sugar, and cinnamon.

Sprinkle the walnut mixture evenly over the cake batter in the prepared pan.

Pour the batter into the greased baking pan and smooth the top with a spatula.

Bake in the preheated oven for about 30 minutes, or until a toothpick inserted into the center comes out clean.

Remove the cake from the oven and let it cool in the pan for about 10 minutes.

Transfer the cake to a wire rack to cool completely before slicing.