Combine sugar, yeast, and water. Stir to dissolve the yeast. Stir in flour 1/2 cup at a time. Stir in 1-1/2 tsp of the salt and the onion. Mix well, then knead until smooth and elastic. Place in an oiled bowl, cover, and let rise until doubled. Punch down, then flatten (1 inch think) onto an oiled sheet pan. Brush the top with olive oil. Let rise until doubled, for about 15 minutes. Sprinkly with crushed rosemary and remaining 1/2 tsp of salt. Bake at 400 degrees F until lightly browned, for about 20 minutes.