In a medium saucepan, boil the cubed potatoes until they are tender.
Drain them, then mash them into a smooth puree.
Transfer the puree to a medium bowl, then add the tuna, green onions, anchovy fillets, parsley, and lemon juice.
Season generously with onion and garlic powder, salt, and pepper.
Mix well with a fork, then shape the mixture into 8 croquettes.
Refrigerate for at least 1 hour and up to 48 hours, covered.

Remove the croquettes from the refrigerator 30 minutes before serving, then coat them in flour, beaten eggs, and breadcrumbs.
Reserve them on a large plate.

In a large skillet, heat a generous amount of oil over medium-high heat.
When the oil is hot, reduce the heat to medium, then add the croquettes.
Fry the croquettes for a few minutes on each side or until they are golden brown.
Remove them from the skillet, then reserve them on paper towels.

Open the burger buns, then top each half with coleslaw.
Top with a tuna croquette, a tomato slice, and pesto.

Creamy Lemon Pesto
In a bowl, mix all the ingredients well.
Use immediately or cover and refrigerate for up to 24 hours.