Make the Dough:
Mix the yeast, warm water and 1 cup (120 g) of the flour in a medium bowl and let sit for 1 hour until bubbles form.
Mix the sugar with the hot water and oil in a small bowl. Add this to the yeast mixture along with the remaining 3½ cups (420 g) flour and knead until smooth, about 12 minutes. Cover and let rise for 1 hour.
Make the Filling: Form the beef into 16-20 small patties. In a skillet, sauté the onions in the butter over medium heat until they are deeply browned, about 15 minutes.
Stuff It!: Divide the dough into 20 uniform pieces. Roll out each piece to 1/8 inch (3 mm) thick, place a patty on it, top with the onions and a slice of cheese and wrap fully with the dough. Repeat to complete all 20 buns.
Cook It: Steam in batches of 5 or 10 depending on how big your steamer is. Steam over high heat for about 12 to 15 minutes, until cooked through. Serve with ketchup.