Preheat the oven to 350 degrees F.
In a bowl, whisk together the oats, flour, baking soda, salt, cinnamon and nutmeg.
In a large bowl, whisk together the melted butter and sugar. Whisk in the egg yolk, milk and vanilla extract. Stir the dry ingredients into the wet ingredients – it will take a few minutes to fully incorporate everything, but you don’t want the batter to be super wet. If at any time the mixture seems way too dry, add more milk 1 tablespoon at a time.
Stir in the zucchini and chocolate chips. At this point, I like to stick the batter in the fridge for 30 minutes or so.
Scoop the dough onto a cookie sheet about 2 inches apart. Bake for 12 to 15 minutes, or until golden on the tops and around the edges. Let cool slightly before lifting from the pan and setting them on a wire rack. Cool completely before serving.