I was at Cafe Rouge in London a few months ago and they served a fantastic onion soup... I e-mailed asking for the recipe and this is all they would let me know...
"It is a classic French onion soup including caramelised onions in butter and oil. We add Dijon and grain mustard, fresh thyme, bay leaf, beef and chicken stock and cook until all flavours have merged into one rich, sweet indulgent darkened brown soup with a savoury/ sweet but mellow onion flavour."
The grain mustard and thyme is what really makes the difference, I think.
(I'm still trying to recreate this... I've come close but not close enough.)