This is a quick version of a chicken soup. Whisking in miso adds an earthy flavour and complexity. Use any noodles and/or vegetables that you like in the soup.
Slice the lemongrass across the grain but do not cut all the way through. (It will be easier to remove the lemon grass if it stays in one piece.) In a medium saucepan, sauté the onion and ginger in oil for 4 to 5 minutes, or until soft. Stir in the lemongrass, stock and soy sauce. Bring to a boil over high heat. Reduce heat to low and simmer. Cook uncovered, for 20 minutes, or until the soup has reduced by a quarter.
Increase the heat to high. Stir in the noodles and boil until just tender, about 3 to 4 minutes. Add the miso and remove from heat. Whisk until miso is dissolved. Add the green onions. Season with salt and pepper and discard the lemongrass. Serve soup garnished with pieces of nori. Serves 4 to 6.