Dough: Dissolve the yeast in warm water. Sift the flour and salt in a large bowl and make a dent in the middle. Add the dissolved yeast and stir well, kneading until soft. In a buttered pan, roll out the dough, cover and set aside in a warm place until it doubles in size. Filling: In a large bowl, mix the tomatoes, onions, olive oil, salt and pepper. Spread the mixture evenly over the phyllo dough.