Stir the yeast into the water in a large mixing bowl & let stand until it is creamy, about 10 minutes. With a wooden spoon, stir in the olive oil followed by the flour & salt. Knead by hand until moist & velvety. Place dough in a lightly oiled bowl, cover & let rise until doubled. Meanwhile, saute the onions in margarine over the lowest possible heat until they are soft & transparent, about 40 minutes. Season with salt & pepper & sugar. Leave to cool. When the dough has risen, turn out onto a floured board & shape into a long log. Transfer to a greased baking sheet & join the two ends to make a ring of 15 cm tp 20 cm diameter. Keep the hole in the centre as open as possible. Cover & let rise till doubled. 30 minutes before baking, preheat oven to 200ºC . When the dough has risen, scoop up the onions in a slotted spoon & pile on top of the ring. Bake for 15 minutes, then reduce the heat to 190ºC , brush the exposed surface with more olive oil & bake another 15 to 20 minutes until golden & crispy. Remove & slide onto a wire rack to cool.