In a small bowl, dissolve the yeast and the pinch of sugar in the remaining 1/2 cup lukewarm water until creamy and starting to bubble. Add to the onion mixture. Stir in the flour, 1 cup at a time, just until the dough becomes too stiff to stir. Turn out the dough onto a lightly floured work surface. With the help of a dough scraper, knead in only as much additional flour as necessary to keep the dough from sticking. Knead the dough until smooth, supple and elastic, about 5 minutes.