Step 1Position racks in the top and bottom thirds of the oven and heat to 375 degrees. Line baking sheets with parchment paper.
Step 2In a medium bowl, whisk together the flour, salt, baking powder and nutmeg.
Step 3In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a hand mixer, beat the butter until smooth and creamy, 1 to 2 minutes. Beat in the granulated and brown sugars over medium speed until light and fluffy, 2 to 3 minutes. Over low speed, beat in the eggs, 1 at a time, until each is fully incorporated. Use a spatula to scrape down the sides of the bowl. Slowly add the flour mixture, beating until thoroughly combined, and scraping down the sides of the bowl and paddle as needed. Slowly add the oats and raisins until they are evenly distributed in the dough.
Step 4Using an ice cream scoop, or a spoon and your hands, to form 2-inch balls, drop mounds of the dough onto the baking sheets, evenly spacing them to fit 6 mounds per sheet.
Step 5Bake the cookies for 10 minutes, then rotate the baking sheets from front to back and top to bottom, and continue to bake until the edges of the cookies are golden brown and the centers are puffed and feel set when lightly touched, 10 to 12 additional minutes. The cookies might seem slightly under-baked, but they will crisp as they cool.
Step 6Remove the sheets from the oven and cool completely on a wire rack. Continue baking the rest of the dough (be sure the baking sheets are completely cool before reusing to bake additional cookies).