01 - Preheat oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or lightly grease each cup with oil spray.
02 - In a large mixing bowl, whisk together Greek yogurt, eggs, honey or pure maple syrup, oil, and vanilla extract until the mixture is thoroughly blended.
03 - In a separate medium mixing bowl, stir together whole wheat flour, baking powder, baking soda, ground cinnamon, and salt until evenly combined.
04 - Add the dry ingredients to the wet mixture. Gently fold together using a spatula just until combined, taking care not to overmix the batter.
05 - Gently fold in the diced apples and, if using, the chopped walnuts or pecans.
06 - Evenly divide the batter among the prepared muffin cups, filling each approximately three-quarters full.
07 - Transfer the muffin tin to the oven and bake for 18–22 minutes, or until a toothpick inserted in the center of a muffin emerges clean.
08 - Allow muffins to cool in the tin for 5 minutes, then remove and transfer to a wire rack to cool completely before serving.