In a large Dutch oven on a low - medium heat, bring the olive oil to heat. Add the fennel, onion and garlic to the pot and cook for 5 minutes or until fragrant and translucent. Stir occasionally to avoid the garlic and onion from burning. Add the diced tomatoes to the pot and cook for an additional 5 minutes or until jammy.
Deglaze the pan with white wine and simmer until 80% of the liquid has evaporated. Add the stock to the pot with a pinch of salt and pepper, turn the heat to high and bring to a boil. Once boiling, immediately turn the heat to low and simmer for 10 minutes.
Turn the heat to medium and add the mussels and vongole to the pot. Cover with a lid and cook for 5 minutes or until all the shells have opened. Gently shake the pot to encourage the shells to open. Scoop the mussels and vongole out of the pot and remove the meat from the shells. Set to the side.
Add the prawns, fish and octopus and cook for 3 – 5 minutes or until the fish is no longer glassy and the prawns have turned pink. Return the mussels and vongole to the pot and very gently stir through.
Take off the heat and stir through the fresh parsley, and squeeze of lemon. Serve immediately.