Cut tofu crosswise into 6 (2/3-inch-thick) slices. Arrange tofu on several layers of paper towels. Cover with additional paper towels; top with a cast-iron skillet or other heavy pan. Let stand 15 minutes. Remove tofu from paper towels. (An easier method is to freeze completely then defrost. No need pressing.).
Combine 2 tablespoons lemongrass, 2 tablespoons water, soy sauce, and 1 teaspoon sesame oil in a 13 x 9-inch glass or ceramic baking dish. Arrange tofu slices in a single layer in soy mixture, turning to coat. Let stand for 15 minutes.
Combine vinegar and the next 3 ingredients (through salt) in a medium bowl, stirring until sugar and salt dissolve. Add carrot and radish; toss well. Let stand for 30 minutes, stirring occasionally. Drain.
Combine remaining 1 teaspoon sesame oil, mayonnaise, and Sriracha in a small bowl, stirring with a whisk. Cut bread horizontally. Spread mayonnaise mixture evenly on cut sides of bread.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Remove tofu from marinade, and discard marinade. Pat tofu slices dry with paper towels. Add tofu slices to pan, and cook for 4 minutes on each side or until crisp and golden.
Arrange tofu slices on bottom half of bread; top tofu slices with carrot mixture and cucumber slices and cilantro sprigs. Cut loaf crosswise into 6 equal pieces.