Wash hands with soap and water.
Break tuna into small pieces.
Cook onion in butter until tender.
Combine cooked onion, 1/3 cup bread crumbs, eggs, mustard, salt and tuna in a medium sized bowl; mix well.
Shape into 6 burgers. Roll each burger in remaining (1/4 cup) bread crumbs.
Spray skillet with non-stick spray.
Cook burgers on medium heat for 3-4 minutes or until brown on one side.
Turn carefully and cook until brown on other side.
Serve on hamburger buns.