Preheat oven to 300 degrees and line 2 baking sheets with parchment paper.
Beat the butter, granulated sugar, and brown sugar with an electric mixer on medium speed for 2 minutes.
Add eggs, vanilla, and lemon juice. Beat on LOW speed for 30 seconds and then medium speed for 2 minutes, stopping to scrape down the sides of the bowl.
With mixer on LOW speed, add flour, oats, baking soda, salt, and cinnamon. Beat just until blended in. Do not overmix.
Stir in the chocolate chips and walnuts.
Use a spring-loaded ice cream scoop or a 1/4 cup measuring cup coated with cooking spray to scoop about 3 tablespoons of dough for each cookie. Place on prepared baking sheets 2 inches apart.
Bake for 20 to 22 minutes, or until edges are golden. The center should still be soft.
Let cool completely on baking sheets.
Notes
Store in an airtight container at room temperature for 4 to 5 days.