Butter two 9-inch round cake pans. Melt the remaining 3 tablespoons of butter. Turn the dough out onto a lightly floured surface, punch it down and divide in half. Roll 1 piece of the dough out to an 8-by-16-inch rectangle; a long side of the rectangle should be facing you. Keep the other piece covered. Spread half of the onions all over the dough to within 1-inch of the top. Starting at the bottom edge, roll as tightly as possible into a cylinder. Pinch the edge to seal, then brush with half of the melted butter. Cut crosswise into 12 slices, wiping the knife after each slice. Arrange the rolls, cut side up, in 1 of the pans, 9 around the outside and 3 in the center. Wipe the work surface and repeat with the remaining dough, onions and melted butter. Cover the rolls with plastic; let rise until almost touching the rims of the pan, about 1 hour.