Cream together butter, shortening and brown sugar until light and fluffy, about 3 minutes. Mix in egg, milk, and vanilla on low speed until well-combined. Whisk together flour, baking soda, salt, cinnamon, and nutmeg in a small bowl. Slowly incorporate into butter and sugar mixture until well-combined. Stir in oats, raisins and walnuts.
Preheat oven to 350º F. Prepare baking sheets by lining with parchment paper or silicon baking mats. Set aside.