Preheat oven to 375°F (190°C). Slightly oil a 9x9-inch or a 9x11-inch baking dish. For moist, thick oatmeal, use a 9x9 pan and for a dryer and thin layer of baked oatmeal, use a 9x11-inch pan.
In a large mixing bowl, combine all the dry ingredients: rolled oats, flaxseed meal, cinnamon, nutmeg, and baking powder.
In another bowl, combine liquid ingredients: almond milk, apple sauce, vanilla extract, melted coconut oil, and maple syrup.
Stir in liquid ingredients to the dry ingredients until well combined.
Fold in 1/3 cup chopped nuts, dried raisins, and 1 cup chopped apples (keep 1/2 cup for later). Stir well to combine in the batter.
Pour the batter into the prepared baking dish and sprinkle coconut sugar, remaining 1/3 cup chopped nuts, and remaining 1/2 cup chopped apples.
Bake for 40-50 minutes or until set to your liking. For a dryer baked oatmeal, use a large baking dish and bake until a toothpick comes clean (about 50 minutes). For moist oatmeal, remove when the top is golden, and the sides are set, but a pick inserted in the center comes out moist with no crumbs on it. It takes about 40-45 minutes of baking time.
Cool the oatmeal down for at least 40-60 minutes before serving, The baked oatmeal set and get easier to serve after cooling down. To serve slices, refrigerate the cooled baked oatmeal for 3 hours to completely set.
Before serving, combine coconut yogurt, icing sugar, and cinnamon. Drizzle on top of the cooled baked apple oatmeal.
Serve 1 cup per breakfast, rewarm in a microwave or in a saucepan over medium heat, until lukewarm or heat cold. Serve topped with coconut milk, almond milk, or a dollop of your favorite yogurt and a pinch of extra cinnamon.
Store in the fridge for up to 4 days in an airtight container or in the baking dish covered with food wrap to keep the oatmeal moist.
Freeze in an airtight container and thaw in the fridge the day before.