Combine the first 5 ingredients in a shallow, sided dish. Coat the oysters, ensuring they are still a little wet from their natural juices, in the flour/cornmeal mixture, gently pressing it on. Heat the oil in a large skillet over medium-high heat. When hot, add the oysters. Cook until golden brown, about 2 minutes per side. Sprinkle with the lemon juice. Split the buns; spread with the mayonnaise and top with the tomatoes and lettuce. Divide and top with the oysters and serve.