Preheat the oven to 350°F (180°C). Line a 12-cup muffin tin with silicone cups.
Mix oats, baking powder, cinnamon, nutmeg, and salt in a large bowl until well combined.
In another bowl, whisk eggs and honey until smooth.
Add yogurt and milk and whisk to combine.
Add applesauce and whisk to combine.
Combine wet and dry ingredients, and stir gently until just combined.
Fold in the diced apple pieces, making sure they’re evenly distributed throughout the batter.
Divide the mixture evenly into muffin cups.
Top with chopped walnuts or pecans if used.
Bake for 25–28 minutes, until set and lightly golden on top.
Let them cool slightly in the pan for a few minutes before transferring them to a wire rack. Enjoy!