Heat olive oil and butter in a large saucepan until melted.
Add chopped fennel, spring onions, and salt to the pan.
Cover and simmer gently for 5 minutes.
Stir in paprika, ground pepper, whole peppercorns, and tinned tomatoes.
Fill the empty tomato tin with water and add it to the pot.
Cover and simmer for 5 minutes while stirring occasionally.
Add halved potatoes and simmer covered for 5 minutes until nearly cooked.
Discard any open mussels that do not close when tapped.
Add the fish, mussels, and optional scallops or prawns to the pan.
Add tarragon and simmer until mussels open.
Garnish with cherry tomatoes and fennel fronds before serving in bowls.