Position two oven racks towards the center of the oven, and preheat the oven to 350°F. Line two large baking sheets with parchment paper or a silicone baking mat.

In a medium bowl, whisk together the flour, cinnamon, baking soda, salt, and nutmeg. Set aside.

In a large mixing bowl or the bowl of a stand mixer, beat the butter on medium speed until creamy. Add the sugars. Beat until well combined, about 2 minutes.

Add the egg and vanilla, and beat until fluffy, about 2 minutes. Scrape down the bowl, and add the mashed banana. Beat until well combined. Add the flour mixture and beat on low speed just until incorporated. Mix in the oats and chocolate chips. Scoop the dough into roughly 2-tablespoon-sized balls onto the lined baking sheets, spacing the cookies about 2 inches apart. (You will need to bake in multiple batches.)

Bake for 6 minutes, then swap the baking sheet between the oven racks. Continue baking for 6 to 8 minutes or until the cookies are golden brown around the edges and the centers appear dry. (The total bake time will be 12 to 14 minutes.) Let the cookies cool on the pan for a few minutes, then transfer them to a wire rack to cool completely. Continue baking the remaining dough. Cooled cookies can be stored in an airtight container for up to 4 days, or frozen for a few months.