Line two baking sheets with parchment paper and preheat the oven to 300°F (150°C).
In the bowl of a stand mixer fitted with the paddle (or in a large bowl with a hand mixer), beat the softened butter with the granulated and brown sugars on medium speed until creamy, about 2 minutes.
Add the eggs, vanilla, and lemon juice; mix on low to incorporate about 30 seconds, then increase to medium and beat until light and fluffy, about 2 more minutes, scraping the bowl as needed.
Reduce mixer speed to low and add the flour, rolled oats, baking soda, salt, and cinnamon; mix just until combined, about 45 seconds. Do not overmix.
Remove the bowl from the mixer and fold in the chocolate chips and chopped walnuts with a spatula or spoon until evenly distributed.
Scoop portions of dough about 3 tablespoons each and place them about 2 inches apart on the prepared baking sheets.
Bake one sheet at a time in the preheated oven for 20–23 minutes, until the edges are golden brown and centers are still soft.
Remove the cookies from the oven and allow them to cool completely on the baking sheet, about 1 hour.