Finely chop the shallot. Thinly slice the bell pepper.
Heat the oil in large, shallow pan and gently sauté the shallot and pepper pieces for 5 minutes. Add the beans, soy sauce, vinegar and Mirin. Add a drop of water, if needed.
Put a lid on and cook for 3 minutes.
Meanwhile cut the cod into bite-sized pieces.
Put the cod pieces on top of the beans. Season them with salt and pepper and drizzle some sesame oil over them. Put a lid on and steam for 3-5 minutes, over low heat, until the fish has cooked through.
Scatter the coriander leaves on top and serve immediately.