Preheat your oven to 350 degrees and spray a 9 by 13 inch pan with cooking spray.
Spread the mayonnaise, mustard, and pickle relish in the bottom of the buns. Add the hot dogs and place all eight hot dogs in their loaded buns in the pan. (You will be able to fit six side to side the long way and then add the last two end to end the short way.)
Top with the chili, then cheese, then onions. Cover with aluminum foil and bake for 45 minutes or until the cheese is fully melted and the buns begin to crisp.