Cut the trout in large chunks. Heat the oil in a pressure cooker. Saute the carrot for 2 to 3 minutes. Place the trout on top of the carrot and add enough cold water so that it's about 2 inches above the ingredients. Cover, place over a high flame, and bring up to pressure. Reduce the flame to medium-low and cook for 60 minutes. The bones should become soft.

Remove from the flame and let the pressure come down. Remove the lid and place back on the stove over a low flame. Add the pureed miso and simmer, without boiling, for 5 minutes or so. 

Place in serving bowls and garnish with 1/4 teaspoon ginger and 1 to 2 teaspoons chopped scallion.