Thinly slice the onion and garlic.
Heat the oil in a pan and soften the onion (for about 5 minutes).
Meanwhile, slice the peppers into 1cm thick strips, then add these to the onion together with the garlic.
Gently sauté under cover for 5 minutes, then add the vinegar, chilli flakes (if used) and tomatoes. Cover and cook for 20 minutes until tender.
Meanwhile, cut the fish into bite-sized pieces.
When the peperonata is ready, stir in the drained beans and put the fish pieces on top. Season with salt and pepper, then put the lid on.
Turn up the heat slightly and cook for 5 minutes. Don’t overcook the fish!
Scatter the chopped herbs on top just before serving.