Peel, core, and dice the apples into small chunks. Set aside.
In a large mixing bowl, combine the oats, brown sugar, cinnamon, nutmeg, baking powder, and salt. Stir to distribute the dry ingredients evenly.
In a separate bowl, whisk together the eggs, melted butter, buttermilk, and vanilla extract until smooth.
Pour the wet mixture into the dry ingredients and gently fold together with a spatula until just combined. Do not overmix to keep the batter light.
Gently fold in the diced apples, distributing them evenly throughout the batter.
Spoon the batter into a prepared muffin tin, filling each cup about 3/4 full. Gently tap the tin on the counter to remove air bubbles.
Bake in the preheated oven at 180°C (350°F) for 20-25 minutes, or until the muffins are golden brown and a toothpick inserted into the center comes out clean.
Remove the muffins from the oven and let them rest in the tin for 5 minutes. Then transfer to a wire rack to cool slightly.
Serve the muffins warm or at room temperature, perhaps with a smear of butter or honey for extra comfort.
