In a large heavy pan, heat the oil and cook the onion till soft and translucent. Next add the garlic and fennel seeds and cook for a further minute. Add the white wine and leave it to bubble for a few minutes.
Add the tomatoes and saffron, if using, and stir well. Add the sugar. Warm the fish stock and pour in. Bring to a boil and then reduce the heat. Place a lid over the pan and leave the chowder to simmer for 45 minutes to let all the flavours develop.
Add the prawns and the fish mix and cook for a few minutes till the fish is cooked through. Pour in the lemon juice, then add the cream, and cook for a further 2-3 minutes. Season well. Whole prawns and tarragon can be used as a garnish if desired. Serve immediately with crispy croutons.