Preheat your oven to 180°C (350°F) and line a muffin tin with paper liners or lightly grease it. Gather your ingredients to keep everything within reach.
Peel, core, and dice the overripe apples into small chunks. Toss them with a squeeze of lemon juice to prevent browning. Set aside.
In a large mixing bowl, combine the rolled oats, flour, brown sugar, cinnamon, nutmeg, baking powder, and salt. Whisk until evenly blended.
In another bowl, whisk the eggs until slightly frothy. Add the melted butter, milk, and vanilla extract, then whisk again until smooth.
Pour the wet mixture into the dry ingredients, folding gently with a spatula until just combined. The batter will be thick and slightly lumpy.
Gently fold in the diced apples, distributing them evenly throughout the batter.
Scoop the batter into the prepared muffin cups, filling each about three-quarters full. Tap the tin lightly on the counter to settle the batter.
Bake in the preheated oven for 20-25 minutes, until the muffins are golden-brown on top and a toothpick inserted in the center comes out clean.
Remove the muffins from the oven and allow them to cool in the tin for 5 minutes. Then transfer to a wire rack and cool slightly before serving.
Enjoy these warm or at room temperature, appreciating their cozy aroma and tender, caramelized apple bits in every bite.
