Finely chop the onion, celery, carrot and red pepper.
Heat 1 tbsp olive oil in a large pan. (Or spray the base with oil).
Add the chopped vegetables and thyme and gently sauté for 10 minutes or so (under cover) until just tender.
Cut the green beans into 2cm bits, then add to the pan.
Add a splash of water, put the fish on top of the veggies. Season with salt and pepper. Scatter the lemon juice on top then squeeze the lemon juice all over the pan.
Put the lid on and gently steam for another 8-10 minutes (depending on the thickness of the fish).
Serve with the chives scattered on top.