Preheat the Oven - Set the oven rack to the center position. Preheat to 375ºF (191ºC). Line two baking sheets with parchment paper.

Mix the Dry Ingredients - Whisk flour, salt, and baking soda together in a small bowl.

Brown the Butter - Heat butter in an 8-inch skillet over medium-high heat, swirling to melt evenly. Stir and cook, scraping the bottom of the pan with a spatula until the butter is golden brown and nutty in aroma, 1 to 2 minutes.

Mix the Sugars - Transfer browned butter to a large bowl, scraping the bits from the pan. Stir in cinnamon and nutmeg. Add brown sugar, granulated sugar, and oil. Whisk together until combined.

Make the Dough - Whisk in eggs, egg yolk, and vanilla until smooth. Use a spatula to fold in the flour mixture until combined, 1 minute.

Add the Oats and Raisins - Stir in oats and raisins until combined. The dough will be stiff in texture.

Shape the Cookies - Divide the dough into 3-tablespoon (60g) portions. Roll into a ball and place 2" apart on the baking sheet, 8 cookies per pan. Use the bottom of a large measuring cup to press each dough ball into 2 ½-inch wide cookies. Re-form edges into a circle if needed.

Bake - Bake one cookie sheet at a time. The edges should be set, lightly browned on the surface, and the centers soft but not wet, about 8 to 10 minutes. Rotate the pan halfway through.

Let Them Cool - Cool the cookies for 5 minutes and then transfer them to a wire rack. Let cool completely before serving.