In a bowl, mix the flour with all the other ingredients up to and including the pepper. Pour in the water, mix well. Cover and set aside.
Heat the oil in a frying pan, sauté the onion and Swiss chard stems. Add the Swiss chard leaves and onion greens, continue to cook until the leaves begin to wilt.
Pour the batter over the vegetables. Cover and leave to solidify over a low heat for approx. 20 mins.
Mix the tomatoes with the balsamic and parsley, season with salt. Arrange the tomatoes on top of the frittata.