Grease a non-stick cast iron skillet and set aside.
Combine the oats, flour, baking powder, salt and cinnamon in a small bowl and set aside.
In a large bowl, combine butter and sugars until fluffy - about 2 minutes.
Add in the eggs and vanilla, mix well, then add in the lemon juice.
Add the flour mixture in one cup increments until fully combined.
Stir in the chocolate chips, walnuts and coconut and then fold the batter out into the skillet. (You should have approximately 1 cup of batter left over for individual cookies.)
Place the skillet in the oven and bake for 25-30 minutes, until golden brown on top and fully cooked in the center. Watch carefully so it doesn't burn.
Garnish as desired - we used a closed star tip on a piping bag filled with vanilla buttercream to make small dollops around the edge, added cherries, walnuts, and sprinkles and extra chocolate chips and shredded coconut for a fun birthday finish.