For waffles, stir flour, cornmeal, sugar, baking powder, lemon zest and salt to combine. In a separate bowl, whisk buttermilk, oil and egg yolks to combine. Whisk into flour mixture and stir until just blended (just like when making pancakes, do not overmix – a few lumps are OK). Whip egg whites until they hold a soft peak when beaters are lifted and fold gently into waffle batter. Lightly grease a preheated waffle iron and spoon about 1/3 cup of batter onto iron (for a 4-x-4-inch waffle). Close and cook until steam from the waffles dies away, about 4 minutes (depending on your iron). Remove waffles and place on an ovenproof dish, cover, and hold in a low (250 F) oven until ready to serve. Waffles are best served fresh, but if preparing in advance, let waffles cool, freeze, then warm uncovered in a 300 F oven for 20 minutes when ready to serve.