01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or lightly coat with oil spray.
02 - In a large bowl, whisk together Greek yogurt, eggs, honey or maple syrup, olive oil or melted coconut oil, and vanilla extract until well blended.
03 - In a medium bowl, stir together whole wheat flour, baking powder, baking soda, ground cinnamon, and salt.
04 - Gradually add the dry mixture to the wet ingredients, stirring gently until just combined. Avoid overmixing.
05 - Gently fold in diced apples and, if using, chopped walnuts or pecans.
06 - Spoon the batter evenly into muffin cups, filling each approximately three-quarters full.
07 - Bake for 18 to 22 minutes, or until a toothpick inserted in the center of a muffin emerges clean.
08 - Allow muffins to rest in the tin for 5 minutes, then transfer to a wire rack to cool completely.