In a very large mixing bowl, beat butter with an electric mixer on medium to high speed for 30 seconds. Add brown sugar, granulated sugar, cinnamon, baking soda, salt and nutmeg. Beat until combined, scraping sides of bowl occasionally. Beat in egg until combined. Beat in as much of the flour as you can with the mixer. Stir in any remaining flour. Stir in rolled oats and walnuts. If you like, stir in raisins.
Drop dough by rounded teaspoons 2 inches apart onto ungreased cookie sheets. Bake in a 350 degree F oven about 10 minutes or until edges are golden. Cool on cookie sheets for 1 minute. Transfer to wire racks and let cool. Makes about 5 dozen cookies.