On a Hebrew National: Just mustard and onions. Wouldn't want to hide that great dawg flavor.
On any other poser dawgs: Depends on what's offered and what I can get to stay on. Under no circumstances will there be catsup (that's for scrambled eggs) or as a side dish all it's own.
Toasted bun would be nice, but not necessary because sometimes I like the bun to squish.