01 - Preheat oven to 350°F. Line two baking sheets with parchment paper.
02 - Melt butter in a medium skillet over medium heat. Continue cooking, stirring frequently, until butter foams and turns golden brown with a nutty aroma, about 3 to 5 minutes. Remove from heat and let cool for 10 minutes.
03 - In a large bowl, whisk together flour, oats, baking soda, salt, and cinnamon until evenly distributed.
04 - In a separate large bowl, combine browned butter, brown sugar, and granulated sugar. Beat until well mixed. Add eggs and vanilla extract, then beat until smooth and creamy.
05 - Gradually add dry ingredients to the wet mixture, stirring gently just until combined. Do not overmix.
06 - Fold in raisins, diced apple, and chopped nuts if using, distributing evenly throughout the dough.
07 - Scoop tablespoon-sized balls of dough onto prepared baking sheets, spacing them 2 inches apart to allow for spreading.
08 - Bake for 11 to 13 minutes, until edges are golden brown and centers appear just set.
09 - Let cookies cool on baking sheets for 5 minutes, then transfer to a wire rack to cool completely.