Combine the cold mashed potatoes, drained canned tuna, fresh chives, dill, dill pickles, garlic powder, onion powder, paprika and cayenne pepper into a medium sized bowl. Mix with a large spoon until well combined.
Form into 4 equally sized patties (each should weigh about 85g) and then carefully dip each patty in the flour, then in the eggs and finally in the breadcrumbs, reshaping them as needed.
Place the finished croquettes in a plate, cover loosely with plastic film and refrigerate for at least 2 hours, or up to overnight.
Preheat your oven to 400°F, brush a small baking sheet with olive oil and place the refrigerated croquettes on it, then delicately brush their tops with a little bit more olive oil.
Bake in the preheated oven for 25 minutes, or until golden and crispy, flipping once halfway through baking.
Meanwhile, make the tartare sauce by mixing all the ingredients together.
Serve the croquettes promptly, topped with a dollop of the tartare sauce and a sprig of fresh dill, if desired.