Steaming is a gentle, moist method of cooking that minimizes the possibility of overcooking and drying out the crab. Here the crab is steamed with an unusual custard that provides a counterpoint to the crab with its salty flavor and its soft texture. The custard cooks on the same plate as the crab for an exquisite banquet presentation. It is equally good made with clams or lobster. The custard is delicious by itself, too. Without the custard, the cut-up crab may be steamed with the same seasonings as steamed whole fish or in black bean sauce.