Preheat oven to 350 F (180° C). Mix mashed potatoes, 3 eggs, tuna, green onions, dill, lemon rind, mustard and chili flakes (if using) and blend well. Form mixture into 8 equal patties, 1 1/4 to 1 1/2 inches (3 to 4 cm) thick. Lightly whisk remaining egg with water in a shallow bowl or pie plate. Place bread crumbs in another pie plate. Lightly dust each patty with flour and dip into egg mixture, coating both sides. Place each patty in bread crumbs and coat both sides. Place patties on a tray and spray them lightly with cooking spray. Bake patties until tops are golden, about 10 minutes. Flip patties and lightly spray with cooking spray: continue baking for about 10 minutes. Patties are done when they are heated through and coating is golden.
Serve with a dollop of tartar sauce if desired.