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.
Recipe courtesy of Sandra Lee