In large bowl, dissolve yeast and sugar in water; let stand for 5 minutes. Add oil and salt. Stir in flour, 1 cup at a time, until a soft dough forms.

Turn onto a floured surface; knead until smooth and elastic, 2-3 minutes. Place in a greased bowl, turning once to grease the top. Cover and let rise in a warm place until doubled, about 45 minutes. Meanwhile, cook beef and onion over medium heat until beef is no longer pink, breaking meat into crumbles; drain.