Preheat your oven to 350°F and line a sheet tray with parchment paper. Break the walnuts into small pieces and spread them evenly on the lined sheet tray. Roast the walnuts for about 7 minutes, until you start to smell a nutty aroma. Be sure to check on them frequently to avoid burning.
In a large bowl, combine the sugar and melted butter. Use an electric mixer or a strong hand to beat the mixture for about a minute. This helps to fully incorporate the sugar into the butter, creating a smooth, consistent base for your dough.
Mix in the egg and vanilla extract, continuing to beat until all the ingredients are thoroughly combined. This step ensures that the egg is evenly distributed throughout the mixture, which helps with the overall structure and flavor of the cookies.
Sift the dry ingredients into the wet mixture, then fold them in with a rubber spatula. Only about half of the flour should be absorbed into the dough at this stage. This initial folding helps to incorporate the dry ingredients without overworking the dough.
Add the rolled oats, raisins, and roasted walnuts to the dough. Continue to fold with the rubber spatula until the last streak of flour disappears. This ensures an even distribution of the oats, raisins, and nuts throughout the cookie dough.
Preheat the oven to 375 and place a rack in the middle.
Let the dough rest for at least 30 minutes. This resting period allows the oats to absorb the liquid evenly, preventing the cookies from spreading too much during baking. Not resting the dough can result in cookies that are too thin and spread out.
After the dough has rested, use a cookie scoop to divide it into even portions. Roll each portion into a ball with your hands, then gently press down to form thick patties. Place these patties on a lined sheet tray, ensuring they are evenly spaced.
Bake the cookies for 8-10 minutes. Look for a golden brown color around the edges of the cookies. Gently touch the center; if it feels set and springs back slightly, the cookies are likely done. This balance ensures a crispy edge with a chewy center.
Remove the cookies from the sheet tray as soon as they are firm enough to transfer. Place them on wire racks to cool completely. This prevents the cookies from continuing to cook on the hot tray and keeps them crisp. Once cooled, store the cookies in an airtight container for the best texture.