Preheat oven to 300°F and line a large baking sheet(s) with parchment paper.
Cream butter, granulated sugar, and brown sugar in the bowl of a stand mixer on medium speed for about 2 minutes. (Use a hand mixer, food processor, or a fork/whisk if you don't have a stand mixer.)
Add the egg, vanilla, and lemon juice, blending with mixer on low speed for 30 seconds, then medium speed for about 2 minutes, or until light and fluffy, scraping down bowl.
With mixer on low speed, add the flour, protein powder, oats, baking soda, and cinnamon, mixing for about 45 seconds. Don’t over mix.
Remove bowl from mixer and fold in the chocolate chips and walnuts.
Portion the dough into 15 balls weighing ~45 grams each onto the baking sheet about 2" apart.
Bake for 20 to 23 minutes, or until edges are golden brown and center is still soft.