In a medium mixing bowl, whisk together the quick-cooking oats, whole wheat pastry flour, baking powder, cinnamon, kosher salt, and ground nutmeg. Make sure there are no lumps and everything is well combined.
In a small, microwave-safe bowl, melt the unsalted butter. Allow it to cool for a few minutes before whisking in the vanilla extract and the egg. Once smooth, whisk in the honey until you have a well-combined mixture.
Make a well in the center of the dry ingredients and pour in the wet mixture. Using a wooden spoon or a rubber spatula, gently stir until just combined. Be careful not to overmix; the flour should disappear without any lumps.
Fold in the diced apple, ensuring it’s evenly distributed throughout the dough.
Cover the bowl with plastic wrap and place it in the refrigerator for 30 minutes. This helps the dough firm up, making it easier to scoop and bake.
Once ready to bake, preheat your oven to 325 degrees F. Line a large baking sheet with parchment paper or a silpat mat to prevent sticking.
Using a cookie scoop or spoon, portion the dough into generous tablespoons and shape them into slightly flattened balls. Arrange the cookies about 2 inches apart on the prepared baking sheet.
Bake in the preheated oven for 10 to 12 minutes, or until the edges of the cookies appear dry and just beginning to brown.
Allow the cookies to rest on the pan for 2 minutes before transferring them to a wire rack to cool completely.
While the cookies cool, make the glaze by whisking together the powdered sugar, vanilla extract, and 1 teaspoon of milk. Adjust the consistency by adding more milk, one teaspoon at a time, until you reach your desired thickness.
Drizzle the glaze over the cooled cookies, letting it set for a few minutes before serving. Enjoy your delicious Apple Oatmeal Cookies!
