Serve these big sandwiches for a filling lunch or casual supper. If oysters aren’t on hand, these are just as good with fried shrimp.
Preheat the oven to 200 degrees. Place a wire rack on a baking sheet, and place in oven. Split bread loaves lengthwise, and scoop out most of centers. Reserve for another use. Place hollowed-out loaves on wire rack in oven, and warm 20 minutes (until you’re ready to assemble the sandwiches).
Meanwhile, pour vegetable oil to depth of 1-inch in a large Dutch oven and heat to 375 degrees F.
Combine the mayonnaise and next 6 ingredients in a bowl. Combine crackers, salt, and pepper in a second bowl. Whisk together buttermilk and egg in a third bowl.
Dip oysters in buttermilk mixture; dredge in cracker crumbs. Fry in hot oil in 2 batches, 1 to 2 minutes or until golden brown. Drain on a wire rack over paper towels.
Remove bread from oven. Spread cut sides of loaves with mayonnaise mixture; fill with oysters and shredded lettuce.