To the bowl of a stand mixer fitted with the paddle attachment (or a large mixing bowl and hand mixer), add the egg, butter, sugars, vanilla, and beat on medium-high to cream ingredients until light and fluffy, about 5 minutes.

Stop, scape down the sides of the bowl and add the oats, flour, cinnamon, nutmeg, baking soda, optional salt, and beat to just incorporate, about 1 minute.

Using a 2-inch medium cookie scoop (about 2 tablespoons), form dough mounds (I made 20). Place dough mounds on a large plate. Flatten mounds slightly. Cover plate with plasticwrap and refrigerate for at least 3 hours, and up to 5 days, before baking. Do not bake with warm dough; cookies will spread and bake thin and flat.

Preheat oven to 350F, line 2 baking sheets with Silpat Non-Stick Baking Mats, parchment, or spray with cooking spray. Place mounds on baking sheets, spaced about 2 inches apart. I bake 8 to a tray. Bake for 9 to 10 minutes, or until edges are set and tops are barely set, even if slightly underbaked in the center and glossy. Cookies may not appear to be done, but they firm up dramatically as they cool. Baking longer results in cookies with dark or burnt bottoms and that set up too crisp and hard and don't stay soft over time.

Allow cookies to cool on trays for about 5 minutes before transferring to a wire rack to finish cooling. Store cookies airtight at room temperature for up to 1 week, or in the freezer for up to 3 months. Alternatively, unbaked cookie dough can be stored airtight in the refrigerator for up to 5 days, so consider baking only as many cookies as desired and save the remaining dough to be baked in the future.