Preheat oven to 350 degrees.
Cream butter, sugar and brown sugar in a mixing bowl using a stand or hand mixer on medium speed for about 2 minutes.
Add egg, vanilla, and lemon juice. Use the mixer and blend on low for 30 seconds. And then blend on medium for a couple of minutes or until light and fluffy, scraping down bowl.
With mixer on low speed, add flour, oats, baking soda, salt and cinnamon, blending for about 45 seconds. Don’t overmix.
Fold in chocolate chips and walnuts.
Line a cookie sheet with a silicone baking mat. Scoop the cookie dough into balls (about 2 tablespoon and place them on the cookie sheet about 1 1/2 to 2 inches apart. Use wet hands and flatten the top of the cookies. The batter is really sticky. Wet hands work well while handling it.
Bake for 13-17 minutes.
Allow the cookies to cool for 5 minutes before removing them from the sheet. If you handle them too soon they will not firm up and will crumble.
Transfer the cookies to a wire rack to cool for an additional 10 minutes.
Sprinkle with sea salt if you wish.