1) In small bowl, sprinkle yeast and sugar evenly over the lukewarm water; stir until yeast dissolves. Let stand 10 minutes or until foamy.
2) In large bowl or in food processor, combine flour and yeast mixture and mix well, or process 1 minute. If mixing by hand, sprinkle work surface with the reserved 1 tablespoon flour; turn dough out onto work surface and knead until dough is smooth and elastic, about 10 minutes.
3) Spray large bowl with nonstick cooking spray; place dough in bowl. Cover loosely with plastic wrap or a damp towel, and let rise until dough triples in volume, about 3 hours. Punch down and wrap in plastic until ready to use.
4) Line steamer rack with parchment paper.
5) Place in large shallow saucepan or wok; add some water, cover and bring to rolling boil.
6) Arrange 8 of the buns at least 1-inch apart on prepared steamer rack; cover and steam until puffy, tender and cooked through, 15-20 minutes.
7) Repeat with remaining buns, adding water to pan as necessary.
8) Move the rolls to a serving plate, sprinkle with chopped green onion and serve.