Preheat oven to 350º F. Prepare baking sheets by lining with parchment paper or silicon baking mats. Set aside.
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.
Drop by the tablespoon onto prepared baking sheets. Bake between 12-15 minutes, until lightly golden brown and set. Remove from oven and allow to cool on pans for about 2 minutes before removing to a wire rack to cool completely.