Preheat oven to 325 degrees F. Line a baking sheet with parchment or aluminum foil.
Mix chili with the onion in a small bowl.
Slice sub rolls open from the top (as in the photo)
Place a hot dog in each roll and divide the chili mixture evenly between the 4 rolls.
Cover pan loosely with foil and bake for 30 minutes or until the hot dogs are hot; discard foil cover.
Divide the shredded cheese evenly over the chili dogs. Place a slice of American cheese on top of each. Bake for an additional 10-15 minutes or until cheese is melted and bubbly.