Preheat oven to 350°F (175°C). Prepare a baking dish (around 8x8 inches or a similar size).
Mix dry ingredients: In a large mixing bowl, combine rolled oats, ground cinnamon, ground nutmeg, baking powder, ground flaxseed (if using), and salt. Stir well to ensure the spices are evenly distributed.
Combine wet ingredients: In another bowl, whisk together the unsweetened plant-based milk, applesauce, maple syrup, and vanilla extract.
Add the apples: Fold in the diced apples into the wet mixture.
Combine: Pour the wet mixture into the dry ingredients and stir until everything is well incorporated.
Transfer: Transfer the mixture to the prepared baking dish and spread it evenly. If desired, sprinkle the chopped walnuts or pecans, apple pieces, and date sugar on top.
Bake in the preheated oven for 40-45 minutes or until the top is golden and the oatmeal is set.
Serve: Let it cool for a few minutes before slicing. Serve warm, either on its own or with maple syrup.