Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan or line it with parchment paper.
In a large mixing bowl, combine the rolled oats, applesauce, almond milk, eggs, vanilla extract, baking powder, cinnamon, and salt. Mix until well combined.
Gently fold in the diced apple, walnuts, and raisins if using. If you prefer a sweeter cake, add the maple syrup or honey at this stage.
Pour the batter into the prepared cake pan and spread it evenly.
Bake in the preheated oven for 30-35 minutes, or until the top is golden and a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.