1. Preheat the oven to 400°F.
2. Place the potato purée in a bowl; mix in the tuna, corn kernels, and parsley.
3. Season with salt and pepper to taste; shape into 8 patties.
4. Spread the breadcrumbs on a plate, and press the patties onto the crumbs to coat lightly; place on a baking sheet lined with parchment paper. Turn off the oven and turn on the broiler.
5. Cook the fish patties under the moderately hot broiler until they are golden brown, turning once. Garnish with lemon slices, and serve with vegetables or a salad.