Preheat oven to 300°F. Line two baking sheets with parchment paper.
In a small bowl, whisk the lemon juice and milk and set aside.
In a large mixing bowl, cream the vegan butter with the sugar and brown sugar for ~2 minutes, until light and fluffy.
Add milk mixture and vanilla to the butter mixture, beating again until light and fluffy.
Add the flour, baking soda, salt and cinnamon. Mix until fully combined.
Fold in the oats, chocolate chips and walnuts.
Scoop out 1/4 cup of dough for each cookie, roll into balls and place onto the parchment lined trays. Leave room in between each for spreading. Slightly press down each ball.
Bake for 20-22 minutes, until edges are golden brown. If desired, sprinkle with flaky sea salt. Allow to cool then enjoy!