Preheat oven to 375. Line a 13x9 baking pan with foil. Place buns inside.
Squeeze desired amounts of mustard, ketchup and relish inside of each bun. Then, put a hot dog on top. Ladle 1/3 cup of chili onto each hot dog, then sprinkle with cheese and onion. Cover pan with foil. Bake for 30 minutes.
Let stand for 5 minutes and then carefully remove foil. Remove hot dogs with a spatula.