Preheat oven to 350F. Line two baking sheets with parchment paper and set aside.
In a medium skillet, melt butter over medium heat, swirling pan occasionally until butter is a deep golden brown and fragrant, about 3-5 minutes of cooking.
Place browned butter and sugars in the bowl of a large stand mixer, fitted with a paddle attachment OR in a large mixing bowl with a wooden spoon. Mix on medium speed or by hand until creamy. Add egg and vanilla, beating well until smooth.
In a separate medium bowl, combine flour, oats, baking soda, salt, nutmeg, and cinnamon. Fold into butter mixture until dough is just combined, then fold in pecans and chocolate.
Use a medium cookie scoop to scoop dough out into 1-in balls. Place balls 2 inches apart on prepared baking sheets and flatten balls slightly with palm of hand.
Bake cookies at 350F 10-12 minutes until edges of cookies are golden brown (centers will still be a little soft - this is ok) Allow cookies to cool 10 minutes on sheet pan then transfer to a wire cooling rack to cool completely. Enjoy!