In a large pot or Dutch oven, heat the olive oil over medium heat. Add the diced onion and cook until softened, about 5 minutes. Add the minced garlic and cook for an additional 1 minute, until fragrant.
Stir in the diced tomatoes, white wine, fish stock, dried oregano, dried thyme, and bay leaf. Season with salt and pepper to taste. Bring the mixture to a simmer and let it cook for about 30 minutes to allow the flavors to meld together.
Once the broth is simmering, add the mixed seafood to the pot. Cover and cook for 10 minutes, or until the clams and mussels have opened and the squid is cooked through. Discard any clams or mussels that do not open.
Taste the broth and adjust the seasoning if needed. Remove the bay leaf from the pot.
To serve, ladle the Cioppino into bowls over cooked polenta or crusty bread. Garnish with chopped fresh parsley and serve with lemon wedges on the side for squeezing over the stew.