01 - Preheat the oven to 350°F. Grease a 12-cup muffin tin or line with paper liners.
02 - In a large bowl, mix rolled oats, ground cinnamon, baking powder, and salt until evenly combined.
03 - In a separate bowl, whisk together eggs, milk, maple syrup or honey, melted butter or coconut oil, and vanilla extract until smooth.
04 - Pour the wet mixture into the dry ingredients and stir gently just until combined.
05 - Carefully fold in peeled, diced apples and optional walnuts, pecans, raisins, or dried cranberries if using.
06 - Divide the batter evenly into the prepared muffin cups, filling each nearly to the top.
07 - Bake for 25 to 28 minutes until the oatmeal cups are set and lightly golden on top.
08 - Let muffins cool in the pan for 5 minutes before transferring to a wire rack to cool completely.