- Combine rice vinegar, sugar, and water in a saucepan. Bring to a boil over medium heat, stirring until sugar has dissolved, about 1 minute. Allow the mixture to cool.
- Place carrot, radish, and onion in a bowl. Pour in the cooled vinegar mixture and let marinate for at least 3o minutes.
- While vegetables marinate, set an oven rack about 6 inches from the heat source and preheat the broiler. Lightly oil a slotted broiler pan.
- Sprinkle chicken breast with garlic salt and pepper.
- Place chicken on the prepared pan and broil, turning once, until browned and no longer pink in the center, about 6 minutes per side.
- Place chicken on a cutting board. Cut into bite-sized pieces.
- Slice the baguette in half lengthwise. Pull out the soft center of the bread, leaving a cavity for the fillings.
- Lightly toast the baguette under the broiler for 2 to 3 minutes.
- Drain off excess vinegar mixture after vegetables have marinated.
- To assemble the sandwich, spread each half of the toasted baguette with mayonnaise. Layer chicken on the bottom half of the bread. Top with cucumber, drained pickled vegetables, cilantro, and jalapeno.
- Squeeze lime wedge over fillings and cover with the top half of the baguette.
- Cut into two 6-inch sandwiches to serve.