Pour olive oil into a large saucepan set over medium-high heat.
When the oil is hot but not smoking, cook the onion until lightly browned, approximately 5 to 7 minutes.
Add the garlic and sauté for 2 to 3 minutes.
Pour in the wine and reduce the heat slightly.
Simmer the mixture for 3 to 4 minutes.
Add the diced tomatoes and herbs.
Pour in the fish stock or clam juice.
Add the clams and mussels (if using). As soon as the clams and mussel shells open, add the shrimp and simmer until shrimp are pink. (If you're not adding clams or mussels, add shrimp right after you've added the fish stock.)
Season with salt and pepper and serve hot.
Add a toasted slice of a thick, hearty bread.