Preheat your oven to 375°F. While it heats, dice the rhubarb into 1/2-inch pieces, chop the walnuts into 1/4-inch pieces, and measure out all dry ingredients. I like to prep the rhubarb first and set it aside on a paper towel to absorb excess moisture—this prevents the cookies from becoming too wet. Having everything measured and ready before you start mixing ensures a smooth, efficient baking process.

In a large bowl, combine the softened butter, packed brown sugar, and sugar. Beat together until the mixture is light, fluffy, and pale—this typically takes 2-3 minutes with an electric mixer. Creaming incorporates air into the dough, which helps the cookies rise and achieve a tender texture. Make sure to scrape down the sides of the bowl to ensure even mixing.

Add the room-temperature eggs one at a time to the creamed butter and sugar mixture, beating well after each addition. This prevents the batter from becoming curdled or separated. Once all eggs are incorporated, add the vanilla and mix until fully combined. Room-temperature eggs emulsify better with the butter, creating a smoother, more uniform batter.

In a separate bowl, whisk together the flour, baking soda, cinnamon, nutmeg, and salt. Whisking ensures the leavening agents and spices are evenly distributed throughout the flour, preventing dry pockets in the baked cookies. This step only takes a minute but makes a significant difference in the final texture and flavor consistency.

Fold the dry ingredient mixture from Step 4 into the wet mixture from Step 3 using a spatula or wooden spoon. Mix until just combined—do not overmix, as this can develop gluten and make the cookies tough. The batter should look thick and slightly textured at this point.

Gently fold in the oats, prepped rhubarb (drained of excess moisture), chopped walnuts, and raisins until evenly distributed throughout the dough. I like to add the rhubarb last and fold it in gently to minimize breaking the pieces apart. The dough will be thick and chunky—this is exactly what you want.

Drop spoonfuls of dough onto ungreased baking sheets, spacing them about 2 inches apart. Gently flatten each mound slightly with the back of a spoon to create an even thickness—this helps them bake uniformly. Bake for 9-12 minutes, until the edges are set but the centers still have a slight softness. The cookies will continue to set as they cool, so remove them from the oven while they're still just slightly underdone in the very center.