Step 1
To a large skillet set over medium-high heat, add the oil, celery, garlic, onion, and chile flakes, if using. Cook, stirring occasionally, until the vegetables are soft, about 10 minutes. Stir in the cuttle-fish, squid, tomatoes, and 2½ cups of water. Season lightly with salt, bring to a boil, then turn the heat down to simmer until the cuttlefish and squid are tender, 20–40 minutes (large cuttlefish may take longer to cook). Add the fish, clams, mussels, shrimp, and prawns and cook until the bivalves open, 3–5 minutes.
Step 2
Drizzle the stew with olive oil, season to taste with salt, garnish with parsley, and serve hot with bread on the side.