Preheat oven to 375 degrees F. Line a 9×13 baking pan with tin-foil (for easy clean-up). Place hot dog buns inside. (I only fit 7, but if you squeeze them together all 8 should fit)
Squeeze desired amounts of mustard, ketchup and relish into the inside bottom of each bun. Then put a hot dog inside each bun, over the sauces.
Ladle about a 1/3 cup of chili onto each hot dog, and then sprinkle with shredded cheddar cheese and onion.
Cover pan with foil. Bake for 30 minutes.
Let stand 5 minutes and then carefully remove foil. Use a spatula to remove chili dogs from pan. Serve warm and enjoy!