Evenly spread 2 tablespoons of mayonnaise and 1 tablespoon of relish into each hot dog bun, top with the (uncooked) hot dog, and place in a 9x13 baking dish.
Repeat for remaining hot dogs.
Evenly top each with chili, then cheese, then diced onion.
Cover with foil, and bake at 350 degrees for 45 minutes.