Scrub live mussels under cold running water. Using your fingers, pull out the beards that are visible between the shells. In a very large bowl, combine 4 quarts (16 cups) of the cold water and 1/3 cup of the salt. Add mussels; soak for 15 minutes. Drain in a colander. Rinse mussels, discarding water. Repeat twice more, using fresh, salted water each time. Rinse well. (If using shrimp, omit this step.)

In a Dutch oven, combine chicken broth and the 1-1/2 cups water; bring to boiling. Add mussels; reduce heat. Cover and simmer for 5 to 7 minutes or until shells open and mussels are cooked through. Discard any mussels that do not open. (If using shrimp, simmer for 2 to 3 minutes or until shells are pink and shrimp is opaque.) Discard any mussels that do not open.

Strain cooking liquid through a cheesecloth-lined sieve into a large bowl; set aside. Set mussels or shrimp aside until cool enough to handle. Remove meat from mussel shells and set aside; discard shells. (Or peel and devein shrimp; discard shells.)

In a large saucepan, heat oil over medium heat. Add leeks and garlic; cook for 3 to 5 minutes or until tender. Stir in reserved cooking liquid, the saffron, and pepper. Bring to boiling; reduce heat. Boil gently, uncovered, about 15 minutes or until reduced to 3 cups. Stir in half-and-half; heat through.

Just before serving, stir mussels or shrimp into soup. Sprinkle individual servings with basil. Makes 8 (2/3-cup) servings.

If using shrimp, add 1/8 teaspoon salt to the soup.

To clean leeks, remove root end and remove any heavy dark green portions. Slice lengthwise and submerge in a bowl of cool clean water. Gently open layers and rinse thoroughly to remove any grit or sand. Repeat as needed.