To the bowl of a stand mixer (or a mixing bowl using a hand mixer) cream the butter, granulated sugar, and brown sugar on medium speed until well combined. About 2 minutes.
Add in the eggs, vanilla, and lemon juice, mixing for about 30 seconds on low speed. Up the speed to medium for an additional 2 minutes until the mixture is light and fluffy.
On low speed, add in the flour, oats, baking soda, salt, and cinnamon. Mix until combined, about 35-45 seconds. Be careful not to overmix.
Fold in chocolate chips and walnuts with a wooden spoon.
Line a baking sheet with parchment paper or a silicone baking mat. Using a large cookie scoop (3 Tbsp), scoop dough balls onto baking sheet.
Bake for 20 minutes, or until cookies are golden brown with a soft center.
Allow to cool on the baking sheet for 2 minutes. Transfer cookies to a wire cooling rack. Enjoy!
To correctly soften butter, remove it from the refrigerator and cut it into tablespoons. Allow it to sit on the counter for 20 minutes.
Be sure to measure your flour properly to avoid dry and crumbly cookies. To do so, scoop your flour into a measuring cup using a spoon, leveling the top gently with the flat end of a butter knife. Scooping flour directly with a measuring cup packs the flour into the cup, in which you will end up with too much.
Do not overmix your cookie dough. This can cause hard, flat cookies. Mix enough to combine the ingredients well.
The acidity in the lemon juice will react with the baking soda, creating bubbles that help with the rise of the cookie.
Oven temperatures may vary, which will affect the cook time of your cookies. Bake until the cookie edges are lightly browned, maintaining a soft center. 20-23 minutes.
If you use a smaller cookie scoop (we used a 3 Tbsp scoop), keep an eye on the cookies in the oven - they will be done sooner than a larger cookie will be.