Scrub clams and mussel shells well with a brush. Discard opened or cracked clams and mussels. Combine black beans and sherry.
Saute onions, garlic, and ginger in hot oil in a large skillet over medium-high heat 2 minutes. Add clams, mussels, bean mixture, and soy sauce. Bring to a boil; cover and cook 5 minutes or until shells open, stirring once. Immediately remove opened clams and mussels with a slotted spoon. Continue cooking remaining clams and mussels for 2 minutes; others may open. Discard any unopened shells.
Bring pan juices to a boil; cook over medium heat until mixture is reduced to 1/2 cup. Transfer to a small bowl; cover and refrigerate.
Break apart and discard the empty top shells of clams and mussels. Using a paring knife, release the mollusk from the remaining shell. Place shells on a platter, and cover tightly with plastic wrap. At serving time, spoon a teaspoon of the bean mixture onto each mollusk, and garnish, if desired.
Note: The clams and mussels and the sauce can be prepared 1 day in advance.