Fillets, steaks, and whole fish are equally steamable, either alone or en papillote. For the most fragile fillets, cooking en papillote eliminates the problem of fish transfer. Steaming is particularly well suited to lean fish; the steam keeps them moist.
Steam scallops and shrimp like fish. For mussels and clams, start by sautéing onions, shallots, garlic, and herbs in a little oil. Add wine, beer, stock, and/or tomatoes, bring to a boil, and add the shellfish directly to the pot (no rack necessary). When they open, serve them with the steaming liquid.