Preheat oven to 200°. Place a wire rack on a baking sheet and put in the oven.
Split the bread lengthwise, and scoop out most of the fluffy center. Place the hollowed-out bread in the oven.
Pat the oysters dry. Put the all-purpose flour, mustard, and corn flour in separate shallow bowls or plates. Pour oil to a depth of about 1/2 inch in a deep skillet, sauté pan, or Dutch oven. Place over medium-high heat and bring the oil to 365°.
Using one hand to handle the food, dredge the oysters in the all-purpose flour, shaking off any extra. Then dip them in the mustard, and use a pastry brush to paint them evenly with a thin layer of mustard. Place the mustard-coated oysters in the corn flour, and dust them all over. As each oyster is coated, set it aside on a sheet of wax paper.
When the oil reaches 365°, add the oysters, using tongs, and fry until golden brown all over, about 1 1/2 minutes on each side. Do not crowd the skillet. As they are done, transfer the oysters to the wire rack to keep warm and drain.
Remove the bread from the oven and spread both sides thickly with tartar sauce. Then line them with lettuce.
When all the oysters have been fried, add them to the sandwiches and serve immediately.