In a medium bowl, whisk the flour, oats, cinnamon, baking soda, nutmeg, and salt together. Set aside.
1½ cups all-purpose flour, 2 cups old-fashioned rolled oats, 1 tablespoon ground cinnamon, ½ teaspoon baking soda, ¼ teaspoon ground nutmeg, ¼ teaspoon kosher salt
Add the butter and sugars to the bowl of a stand mixer fitted with the paddle attachment. Beat together until well mixed, about 2 minutes.
½ cup unsalted butter, ¾ cup brown sugar, ¼ cup granulated sugar
Add the egg and vanilla extract and beat until fully incorporated.
1 large egg, 2 teaspoons pure vanilla extract
Pour the dry ingredients into the wet ingredients and beat just until mixed. Do not overmix.
Stir in the diced apple.
1 cup finely diced apple
Line 2 baking sheets with parchment paper. Set 1 aside. Use a 2-tablespoon-sized cookie scoop to scoop cookies onto the baking sheet. Try to fit all of the cookie dough balls in a single layer on the baking sheet. If the cookies aren’t holding their round shape, use your hands to press the dough balls together to form round cookies. Do not flatten cookies.
Once all of the dough has been scooped, place the baking sheet of cookie dough balls in the freezer for 10 minutes. This prevents the cookies from spreading too much while they bake.
While the cookie dough chills, preheat oven to 350°F.
Once the dough has chilled, place some of the chilled cookie dough balls 2 inches apart onto the second prepared baking sheet.
Bake for 10-12 minutes, or until the cookie edges are just set and bottoms are slightly browned.
Let cookies cool completely on the baking sheet before storing. Repeat until all cookies are baked. Serve and enjoy!