Mix together all the dough ingredients and knead until a smooth and elastic dough forms. Cover and let the dough rise for 3 hours.
Meanwhile, prepare the filling: Heat olive oil in a skillet over medium heat and add the diced onions with a pinch of salt. Cook, stirring occasionally, until the onions turn golden and soft. Add the butter and continue cooking, stirring frequently, until the onions are deeply caramelized and rich brown. Stir in the miso paste and mash everything together until fully combined and smooth. Set aside and allow it to cool completely.
Divide the rested dough into two even pieces. Roll each piece into a long strip, about 8-10 inches in length. Spread a generous amount of the caramelized onion miso butter on each strip, leaving about 1 inch at the edges free of spread for sealing. Carefully roll up the dough and pinch the ends to seal.
Place the rolls spiral side up on a baking pan. Press each dough down to flatten. Cover with plastic wrap and allow it to proof in a cool place (about 65-70F) for 8-10 hours.
Preheat the oven to 425F. Bake the proofed dough for 25 minutes or until the rolls have expanded and reached a golden brown.