Heat the oven to 200°C/fan 180°C/gas mark 6. Heat the butter in a large saucepan over a medium heat. Add the onion, leek, celery and fennel and cook over a medium/low heat for 15 minutes, until they are completely soft. Meanwhile, leave the saffron to infuse in the hot stock.
Place the pieces of bread on a baking sheet and mix with the oil and seasoning. Bake for 10-15 minutes, until golden brown all over.
Add the garlic, tomatoes and tomato purée to the pan and cook for 5 minutes, then add the wine and saffron-infused fish stock with seasoning. Add the potatoes and simmer for 15-20 minutes, until just cooked, then add the salmon and prawns and cook for a further 5 minutes, until just cooked. Stir through the cream and dill.
Divide the soup between 6 bowls, then top with the croutons, dill and a dollop of sour cream.