Place the carrot in a small bowl with the parsley, lemon juice, and salt and mix to combine. Set aside.
Drain the tuna and place it in a small bowl with the mayo, soy sauce, and scallion. Mix well, breaking up the chunks with a fork, taste, and adjust with an optional pinch of salt if desired. Set aside.
Cut a piece of plastic wrap slightly larger than a nori sheet, and place it on a work surface. Lay a nori sheet on top of it.
With wet hands, place ¼ of the rice in the center of the sheet and shape it into a square, pressing it down so it's dense.
Place 5 lettuce leaves on top of the rice and top with half of the tuna mixture.
Top the tuna with half of the carrot mixture.
Place another ¼ of the rice on top of the carrot and fold 2 diagonal corners down snugly over the rice.
Repeat with the other two corners and wrap the onigirazu snugly in the sheet of plastic wrap it's sitting on.
Repeat steps 3-8 with the other half of the ingredients to make the second onigirazu. Set the wrapped onigirazu aside for about 5 minutes to allow the nori to soften.
Unwrap the onigirazu and cut them in half with a wet knife before serving.