Preheat your oven to 350 F (175 C). Line a muffin tin with paper liners or grease it lightly.
In a large mixing bowl, whisk together the all-purpose flour, rolled oats, granulated sugar, brown sugar, baking powder, baking soda, salt, ground cinnamon, and ground nutmeg.
In a separate bowl, whisk together the eggs, applesauce, melted butter, and pure vanilla extract until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined. Be careful not to overmix; a few lumps are okay.
Gently fold in the diced apples until they are evenly distributed in the muffin batter.
Using a scoop or spoon, divide the muffin batter evenly among the muffin cups, filling each cup about 2/3 full.
Sprinkle sliced nuts (such as almonds, walnuts, or pecans) on top of each muffin. Gently press the nuts into the muffin batter so that they adhere.
Bake in the preheated oven for 20-25 minutes or until the muffins are golden brown and a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the tin for a few minutes before transferring them to a wire rack to cool completely. Once cooled, savor the delightful taste and texture of your Apple Cinnamon Oat Muffins with sliced nuts on top.
