Combine flour, oats, baking soda, salt and cinnamon in a medium bowl. Stir well with a wire whisk. Set aside.
Using an electric or stand mixer, combine sugars on medium speed until well blended. Add shortening on low speed then add butter on low speed to break the butter into chunks. Slowly increase speed and cream completely into sugar until light and fluffy, at least 2minutes.
Add eggs one at a time to the sugar and butter mixture, mixing well after each addition. Mix in vanilla and lemon juice.
Gradually mix in dry ingredients on low speed until well blended. Slowly add the chocolate chips to the cookie dough.
Cover and chill in the refrigerator for at least two hours. (I will admit to skipping this step when there is an immediate need for cookies - so about half the time - but if you have more willpower than I usually do try to chill the dough.)
Preheat oven to 325° F
Scoop cookie dough using a #20 scoop (or slightly less than 1/4 cup) onto a cookie sheet lined with parchment paper or a silicone baking mat. Bake 14-16 minutes or until barely done. They will be light brown on the edges and puffy in the center (if you touch the center of a cookie it will “sigh” if it’s done). They will darken slightly as they sit from the residual heat of the cookie sheet. Do not over bake.