01 - Preheat the oven to 375°F. Line a 12-cup muffin tin with paper liners or lightly coat with nonstick spray.
02 - In a large mixing bowl, whisk together Greek yogurt, eggs, honey or maple syrup, olive oil or melted coconut oil, and vanilla extract until thoroughly blended.
03 - In a separate 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 and stir just until combined. Avoid overmixing to ensure tender muffins.
05 - Gently fold in the diced apples and, if using, the chopped walnuts or pecans until evenly distributed.
06 - Spoon the batter evenly into the prepared muffin cups, filling each about three-quarters full.
07 - Place the muffin tin in the oven and bake for 18 to 22 minutes, or until a toothpick inserted into the center of a muffin emerges clean.
08 - Allow the muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely before serving.