Peel and chop the potatoes into chunks, then place them in a large pot of lightly salted boiling water and cook until tender, about 6–8 minutes.
Drain the potatoes and mash them with a fork or masher until mostly smooth; set aside to cool completely.
Preheat the oven to 180°C (350°F) and line a baking sheet with parchment paper.
Place the drained, flaked tuna in a large mixing bowl. Add the cooled mashed potatoes, chopped parsley, chopped capers, and lemon zest.
Season the mixture with sea salt and black pepper to taste, then mix until evenly combined.
Scoop or form the mixture into even-sized balls using your hands or an ice cream scoop, then roll each meatball in breadcrumbs to coat.
Arrange the breadcrumb-coated meatballs on the prepared baking sheet, brush each with olive oil, and bake for about 10 minutes until golden and heated through.
Remove from the oven and serve warm with lemon wedges or sauce if desired.