Thinly slice the onion. Chop the onion and celery.
Slice the bell peppers.
Heat the oil in a large, shallow pan and gently sauté the onion, garlic and capers for 5 minutes.
Add the sliced peppers and the marjoram, put a lid on and sweat the peppers for 5 minutes. (You can add some balsamic vinegar or white wine with the peppers if you’d like).
Add the passata and the same amount of water and bring to the boil.
Stir in the stock powder. Cook for 5 minutes, then add the cavolo nero or spinach. Put a lid on and cook for another 5 minutes.
Meanwhile, cut the fish into bite-sized chunks, removing the skin, if preferred.
When the greens are cooked and the stew is thick and silky, put the fish pieces and prawns on top of the stew. Season them with salt & pepper, then put a lid on and steam them for 3-5 minutes.
Turn off the heat and rest for another 3 minutes to make sure they are cooked through, then stir, together with some fresh herbs, and serve immediately.