Coarsely grate zucchini, either in a food processor or with the large holes of a box grater (no need to peel them). In a medium bowl, stir grated zucchini with salt. Let stand 10 min. 

Separate eggs. Whisk egg whites in a large bowl, using an electric mixer on high, until stiff, 2 to 3 min. Reserve. 

Transfer zucchini to a colander and rinse well. Using your hands, squeeze out as much of the water as possible. Return to the bowl and mix in egg yolks, feta, green onions and mint until well-combined. Sprinkle flour overtop and mix well. Fold in beaten egg whites. 

Heat oil in a large non-stick fry pan over medium. Working in batches, add a level, 1/4 cup zucchini mixture, flattening into pancake shape. Fill the pan with pancakes. Cook 2 to 3 min per side or until golden. Serve immediately with sour cream alongside, or keep warm on a rack set over a baking sheet in a 200F oven.