Preheat oven to 375 F (190 C).
Coat a non-stick pan with cooking spray, sauté onion for 5 to 8 minutes, or until brown. Remove when done.
Combine leftover tuna (or canned tuna with mayonnaise), eggs, onion, bread crumbs, garlic powder, and celery salt.
Form patties, place on parchment paper covered baking sheet.
Bake 10 minutes on each side. Serve and enjoy!
