Although it's not in Dusseldorf proper, I tend to visit the very likable steakhouse Gut Jaegerhof in Erkrath. Was shown this by local friends and it's worth going for the atmosphere alone.
Good food and a beautiful summer terrace with views over the (well, obviously) historic Neanderthal.
http://www.gut-jaegerhof.de
http://www.gut-jaegerhof.de/site/......sh_all_2mb.pdf