1. Preheat oven at 350°F.
  2. Combine oats, flour, brown sugar, salt, cinnamon powder, and nutmeg in the bowl of a food processor. Process until well combined. Fold in butter and continue to process until a crumbly texture is achieved. Set aside a quarter of the dough for later use.
  3. Press the remaining dough into the bottom of an oven pan lined with baking paper. Thickness should be around 1 inch (2.5cm).
  4. Bake for 15 minutes. Arrange apple slices and crumble the remaining dough on top. Bake for 10 more minutes or until golden brown.
  5. Cut into individual bars or squares. Place in a jar or a storage container. Store for 1-2 days at room temperature and for 2-3 days in the refrigerator.