At least 1 hour before cooking clams, mix the kosher salt and cornmeal into 1 gallon of water and soak the clams. Do not use iodized salt as it will instantly kill the clams.

In a steamer or large pot set over medium-high heat, melt the butter. Add the shallots and leeks and saute until soft, about 5 minutes. Add the remaining ingredients, except bread, and bring to a boil. Reduce the heat to a simmer and add the clams and mussels. Cover with a tight fitting lid and steam over low heat until clams and mussels have opened, about 5 to 10 minutes. Discard any clams or mussels that have not opened.

Transfer to serving bowls and ladle with broth. Serve hot with crusty bread to soak up the broth.