Stir the yeast into a small bowl & let proof for 10 minutes. Stir in the rest of the water & oil. Whisk in 2 c of the flour & stir till smooth. Add salt & the rest of the flour, 1 c at a time. Knead for 8 to 10 minutes until soft & velvety. Place in a lightly oiled bowl, cover & let rise till doubled. While the dough is rising, prepare the topping. Warm the olive oil over low heat & saute the onions for about 20 minutes. Cool to room temperature. Cut the dough into 2 pieces, one twice as large as the other. Shape the smaller one to fit an oiled 25 cm round pie plate & the larger one to fit an oiled 27 cm X 39 cm baking sheet. Cover & let rise for 45 minutes until bubbles appear in the dough but it has not quite doubled. Heat the oven to 200ºC for 30 minutes before baking. Sprinkle the onions over the dough & drizzle with olive oil & finish with the salt. Bake for 25 minutes until the onions are golden. Cool on wire racks. VARIATION: Crescia al Rosmarino. Substitute 2 tb finely chopped rosemary for the onions.