01 - Preheat oven to 350°F and line two baking sheets with parchment paper.
02 - Whisk together flour, cinnamon, ginger, nutmeg, baking soda, and salt in a medium bowl until thoroughly blended.
03 - Beat softened butter with brown sugar and granulated sugar in a large bowl until light and fluffy, approximately 2-3 minutes.
04 - Incorporate eggs one at a time, beating well after each addition, then mix in vanilla extract until fully combined.
05 - Gradually add dry ingredient mixture to wet ingredients, mixing just until flour disappears. Avoid overmixing.
06 - Gently fold in oats, raisins, and diced plums by hand until evenly distributed throughout the dough.
07 - Drop rounded tablespoons of dough onto prepared baking sheets, leaving approximately 2 inches of space between each cookie.
08 - Bake for 13-15 minutes until edges turn golden brown and centers appear just set.
09 - Let cookies rest on baking sheets for 5 minutes before transferring to a wire rack to cool completely.