Heat oven to 375°F and lightly oil 9- by 13-inch baking dish. In large bowl, stir together oats, salt, nutmeg, cinnamon, and baking powder.
In second large bowl, whisk eggs. Add milk, applesauce, maple syrup, and, vanilla extract and whisk until combined. Add dry ingredients to bowl and stir until just combined.
Fold chopped apples into batter
Spread batter in pan in even layer. Bake for 45 minutes until the top is nice golden brown. Remove it from the oven and let it cool for a few minutes.
I prefer serving this warm but can be served room temperature as well. Store in the fridge for 4-5 days.