Set it to 350°F (175°C) and grease an 8×8-inch baking dish with cooking spray or a bit of oil.
In a large bowl, combine the oats, baking powder, cinnamon, nutmeg, and salt. Stir well to evenly distribute the spices.
Prepare the wet ingredients
In a separate bowl, whisk together the almond milk, ground flaxseed (or egg), vanilla extract, and applesauce. If you’re adding protein powder, mix it into this wet mixture to ensure it dissolves properly.
Combine the wet and dry ingredients
Pour the wet mixture into the bowl with the dry ingredients. Stir until fully combined. Gently fold in the diced apple.
Transfer to the baking dish
Spread the mixture evenly in the prepared baking dish, smoothing the top with a spatula.
Arrange your desired toppings over the surface. For a fruity twist, scatter fresh blueberries or sliced bananas. If you love extra crunch, sprinkle chopped nuts on top.
Place the dish in the oven and bake for 35–40 minutes, or until the oatmeal is set and the top is lightly golden.
Allow the oatmeal to cool for 5 minutes before slicing into squares or scooping out portions. Enjoy it warm, or let it cool completely and refrigerate for up to 5 days.