Preheat oven to 350 F (176 C) and set aside a non-stick Angel Food Cake Pan, Do Not spray with with non-stick spray.
Place sugar and lemon zest in a food processor and pulse until it becomes a fine powder.
Combine the egg whites, lemon juice, water, and cream of tartar in stand mixer, then whip on medium speed using the whisk attachment until soft peaks form.
Keeping the mixer on, slowly pour in the lemon sugar as the mixture stiffens more to medium peaks.
Remove bowl from stand mixer and gently fold in flour with a spatula, adding just enough to coat the top at a time.
Move the spatula slow and carefully, DO NOT stir, you want to keep all the air bubbles.
Scoop the batter evenly into the cake pan, don't smooth or flatten.
Place cake pan on middle rack and bake for 30-35 minutes. The exposed cake should turn golden brown and when you insert a skewer halfway down the cake should be dry.
Once baked, flip the cake pan upside down with the open side on the cooling rack.
Let cake cool for about an hour to allow it to drop from the pan. If the cooled cake hasn't dropped, run a small flat knife around the edges then flip upside again.