Heat the oil in a large pan, add the fennel seeds, carrots, celery and garlic, and cook for 5 mins until starting to soften. Tip in the leeks, tomatoes and stock, season and bring to the boil, then cover and simmer for 15-20 mins until the vegetables are tender and the sauce has thickened and reduced slightly.
Add the fish, scatter over the prawns and cook for 2 mins more until lightly cooked. Ladle into bowls and serve with a spoon.