Heat the olive oil in a medium-size sauté pan and add the onions, white parts of the scallions, the garlic, ¼ teaspoon salt, and a pinch of pepper.  Sauté over medium heat until the onions begin to soften, about 3 minutes.  Add the corn and the water, lower the heat, and cover the pan.  Simmer until the corn is tender, about 5 minutes.  Transfer to a bowl, toss in the scallion greens, and set aside to cool. Combine the egg yolks, ricotta, milk, and the cheeses in a mixing bowl.  Stir in the corn mixture, flour, baking powder, ¼ teaspoon salt, and a pinch of pepper.  Beat the egg whites with a pinch of salt until stiff and gently fold them into the batter. Spoon the batter into a generously oiled skillet or griddle over medium-high heat allowing about ¼ cup batter per cake.  Cook for about 3 minutes on each side, until the cakes are golden.  Add fresh oil to the pan between batches of cakes.