This is Greek fisherman's soup, which my family on the Greek island of Skopelos would make with whatever shellfish came in on the boat that day. You might not have a fishing boat loaded with shellfish at your disposal, but don't let that stop you. Provided you add 1 to 1 1/2 pounds of shellfish, you can use any combination you like--shrimp and mussels, mussels and clams, or all three--as long as they're fresh. A simple but heady mixture of tomatoes, white wine, onion, and garlic lets the seafood shine. This recipe can be easily doubled.
1. Pour the olive oil into a large saucepan over medium-high heat. When the oil is hot but not smoking, add the onion slices and cook until lightly browned, 5 to 7 minutes. Add the garlic, saute for 2 to 3 more, then pour in the wine. Reduce the heat slightly and simmer for 3 to 4 minutes. Add the tomatoes and the herbs.
2. Pour in the stock and add the clams and the mussels. As soon as the shells open, 3 to 4 minutes, add the shrimp and simmer just until they are pink, 2 to 3 minutes. Discard any unopened clams or mussels.
3. Add salt and pepper and serve hot. If you like, add a toasted slice of rustic bread beside each serving.
4. Cat's Note: To clean clams, scrub them. Encourage them to purge sand or grit by pouring cold water into a bowl with a teaspoon or two of salt. Add the clams and let them sit for 2 to 3 minutes, then pour out the water, add fresh water and salt, and let them sit another 2 to 3 minutes. Repeat 2 or 3 times.
5. Buy mussels the day you plan to use them. About 1 hour before cooking, scrub them with a brush (no soap!) under cold water to get rid of grit, and pull off the fringed byssus (also called a beard) or scrape it off with a knife. Discard any that are cracked.