It may be heresy, but I prefer a dual line of mustard dressing each side of the dog, giving flavor to both the dog AND the bun.

I have been known to put the chopped onion UNDER the dog (while still maintaining vital dog-onion contact) to avoid spillage in certain places (like a business trip through an airport, or a dog grabbed during lunch off a cart) that I don’t want to risk trashing my shirt should spillage occur.