Preheat your oven to 350°F (175°C). Grease a muffin tin or line it with paper liners to prevent sticking.

In a large mixing bowl, whisk together the whole wheat flour, rolled oats, baking powder, baking soda, ground cinnamon, and salt until well blended.

In another bowl, mix the Greek yogurt, apple sauce, honey or maple syrup, eggs, and vanilla extract until smooth and creamy.

Pour the wet mixture into the bowl with the dry ingredients. Gently fold using a spatula until just combined, being careful not to overmix.

Gently add the diced apples to the batter, mixing until evenly distributed throughout the muffin mixture.

Scoop the batter into the prepared muffin tin, filling each cup about 2/3 full to allow room for rising.

Place the muffin tin in the preheated oven and bake for 18-20 minutes. They are done when a toothpick inserted into the center comes out clean.

Remove the muffins from the oven and let them cool in the tin for 5 minutes before transferring them to a wire rack to cool completely. Enjoy them warm or at room temperature.