Heat the oven to 375°F. Arrange the split buns in a lightly greased 9-by-13-inch baking dish.
Place a halved hot dog in each bun and spoon the warm chili evenly over the top.
Sprinkle with the cheddar and diced onion, then drizzle with mustard.
Cover loosely with foil and bake for 20 minutes. Uncover and bake 8 to 10 minutes more, until the cheese is melted and the buns are crisp at the edges.
Rest for 5 minutes before serving so the chili settles into the buns.
Notes
Model-authored fallback: the source identity supports a coherent chili cheese dog bake; quantities and method are conventional recipe guidance.