Fill a medium pot with water, add a generous pinch of salt, and bring it to a boil over high heat.

Add the potato chunks to the boiling water. Lower the heat to keep the water at a steady simmer and cook for 10 to 12 minutes, until the potatoes are very tender when pierced with a fork.

While the potatoes cook, stir the mayonnaise, tomato paste, and 2 teaspoons lemon juice together in a small bowl until smooth.

Season the sauce with a small pinch of black pepper. Set the bowl aside for serving.

Cut the remaining lemon half into wedges and set them aside.

Drain the potatoes in a colander.

Transfer the drained potatoes back to the warm pot. Mash them until mostly smooth, with just a few small pieces left for texture.

Let the mashed potatoes stand in the pot for 1 minute so some of the steam can escape. This helps the cakes hold together better.

Add the drained tuna to the pot and break it up with a fork.

Add all but 2 tablespoons of the scallions to the pot.

Add the beaten egg, Dijon mustard, 1/2 cup of the panko, 1 teaspoon kosher salt, and 1/4 teaspoon black pepper to the pot.

Stir the mixture until it is evenly combined and holds together when pressed with a spoon. If it feels loose, let it rest for 2 minutes so the breadcrumbs can absorb moisture.