Preheat oven to 250C. Cook potatoes in a saucepan of boiling salted water until tender (10-12 minutes). Drain and keep warm. Just before serving, toss through melted butter and season to taste.
Meanwhile, heat wine in a large saucepan over high heat, crush tomatoes into wine, add garlic and bring to the boil, then add mussels and clams and cook, covered, until mussels and clams open (2-3 minutes). Add prawns, trout and olives and stir gently over low heat until trout is cooked medium-rare and prawns are cooked through (5-7 minutes), then add herbs and season to taste.
Scrunch four pieces of baking paper into bowls to hold seafood. Divide seafood among bowls, gather edges of baking paper together and tie with kitchen string. Serve immediately with potatoes to the side.