Preheat oven to 350 F. In a small bowl, combine mayonnaise and mustard. Evenly spread inside hot dog buns. Place hot dog buns in a large baking dish.
Place one hot dog into each hot dog bun. Pour chili evenly over hot dogs. Sprinkle with shredded cheddar cheese and diced onions. Cover with foil and bake for 35-40 minutes.