In a bowl, prepare the vinaigrette by mixing the soy sauce, brown sugar, mirin, ginger, garlic, onion, sesame oil and chili pepper if desired. Clean the onaga or opakapaka and place in a large pot. Add a half cup of water and a half cup of the prepared vinaigrette. Steam the fi sh for about 20 minutes. Remove from the pot, place on a large platter and pour the vinaigrette on top. Garnish with chopped green onions and cilantro.