Preheat oven to 350 degrees.
In a medium bowl stir together rolled oats, cinnamon, nutmeg, baking powder, and salt. Set to the side.
In another medium bowl crack and lightly beat the eggs. Whisk in the the brown sugar until blended. Whisk in the Greek yogurt, vanilla extract, milk, and coconut oil.
Stir oats mixture into the wet ingredients until combined.
Gently stir in the chopped apple and walnuts.
Spray a 12-cup muffin pan with cooking spray. Drop oatmeal mixture, using a 1/3 measuring cup, into each of the muffin pan cups. Using a spoon press down the oat mixture into the liquid in each of the cups. This is to make sure the liquid is evenly distribute among the oats.
Bake for 30 minutes or until the top is lightly browned.
Take out of the oven and let the oatmeal cups cool for 5 minutes. Remove muffins and serve with pure maple syrup.