01 - Preheat oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or lightly grease each cavity with oil spray.
02 - In a large bowl, whisk together the Greek yogurt, eggs, honey or maple syrup, olive oil or melted coconut oil, and vanilla extract until fully blended.
03 - In a medium bowl, stir together the whole wheat flour, baking powder, baking soda, ground cinnamon, and salt.
04 - Add the dry mixture to the wet ingredients and stir gently until just incorporated, avoiding overmixing.
05 - Gently fold in the diced apples and walnuts or pecans if using.
06 - Evenly distribute the batter among the prepared muffin cups, filling each about three-quarters full. Bake for 18 to 22 minutes or until a toothpick inserted into the center of a muffin emerges clean.
07 - Let the muffins rest in the tin for 5 minutes before transferring to a wire rack to cool completely.