Step 1In a toaster or using a dry frying pan, lightly toast the age tofu until it's crispy and slightly toasted but not dry. Slice crosswise into one-half-inch pieces.
Step 2In a medium saucepan, bring dashi to a boil over medium heat, then reduce the heat to a simmer. Stir in the nappa cabbage and continue to cook for a couple of minutes to wilt the cabbage.
Step 3Add the miso to the dashi, stirring to blend in until the miso is dissolved. (The miso can be dissolved with a little dashi in a separate bowl before adding it into the dashi.)
Step 4Just before serving, top the soup with the sliced crisp tofu, and garnish with sliced green onion.