Preheat oven to 350 degrees. Line baking sheets with parchment paper.
Add bananas to bowl of stand mixer. Mix until mashed and creamy.
Add butter and sugar to bananas and mix again until creamed.
Add egg and vanilla and mix until well beaten, about 2 minutes.
In a separate bowl add flour, ground oats, salt, baking soda, cinnamon, and nutmeg. Stir to combine.
Add flour mixture to banana mixture and mix until combined.
Stir in chocolate chips and walnuts if desired.
Drop cookies onto parchment lined baking sheets using a small cookie scoop, about an 1/8 cup.
Bake 8-10 minutes. Watch carefully so you don't over bake.