First, cut onion in half, then cut each half in quarters lengthwise and slice thinly against the grain.
Cut potato in half and dice half about ¼ inch pieces and put to one side. Cut the other half into large chunks.
Put the larger chunks of potato in a small pot and add 500 ml water and 1 tsp salt. Bring to a boil, simmer until tender then put aside, leaving the potatoes in the water.
Heat the oil in a large saucepan. Add the diced onion and garlic and cook gently until tender but without color.Add the diced fennel and small pieces of potato then cook for a further 4 minutes until it starts to soften and
break down.
Add red chili flakes and white wine and reduce until almost dry.
Next add the 700ml of water and the sliced red peppers.
Strain the water from the cooked potato into the soup mixture, then mash the cooked potato well and stir it into the soup.
Bring the soup to simmer and cook for a further 12 minutes then season to taste with salt and fresh ground pepper.
Season the Norwegian Skrei with salt and pepper then add to soup. Cover the soup and simmer gently for approximately 5 minutes or until just cooked through.
Gently remove the cooked Skrei and place into serving bowls using a slatted spoon then stir basil and spinach into soup to wilt. Finally evenly ladle the stew over the fish into each bowl.
