French term (edited): onions coque

I have already translated this for a local gastonomic restaurant, it was written "onion à la coque" which turned out to be the thick, outer (edible) layer of an onion stuffed with vegetables.

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Note added at 17 mins (2012-05-24 11:28:45 GMT)
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Just remembered it was actually written "coque d'onion", which I think that would most likely be stuffed with vegetables as mine was, rather than puree - especially since there is an "et"... Then again there could be all sorts of things in there, from sausage meat to tofu to creamy mushrooms - so I reckon your best bet is to a) keep it simple and ambiguous as a "stuffed onion", or b) ask your client what's inside. :o)

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Note added at 5 hrs (2012-05-24 16:24:41 GMT)
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My boyfriend is actually a chef, and he worked in the restaurant in question this winter. Here is our skype conversation on the subject:

[13:25:58] Kally Herrison: dit, les coques donion y a quoi dedans? tjs les legumes?
[13:26:31] Niko: Tu met ske tu veux
[13:26:48] Kally Herrison: si cest sur un menu et cest ecrit que onion coque, ca sera surprise alors?
[13:32:47] Niko: non pas forcemen
[13:33:35] Kally Herrison: ca cera vide?
[13:34:15] Niko: en fait on fait cuire les oignons entier dans l eau bouillante
[13:34:28] Niko: et apres on coupe un chapeau
[13:34:43] Niko: et apres on enleve le millieu
[13:35:05] Niko: comme sa on peu le farcire avec se k on veu
[13:35:37] Kally Herrison: donc si ct sur un menu, le clent va ensuite choisir une farce?
[13:36:04] Niko: ouai
[13:36:24] Niko: enfin plutot le chef
[13:37:06] Niko: sur le menu sa sera ecrit coque d oignon farci avec j sais pas koi
[13:38:35] Niko: tu comprend?
[13:40:15] Kally Herrison: mais c esrit que onion coque?
[13:41:32] Niko: ouai mais normallement sa serai coque farci de ...... [13:42:15] Niko: ctai theiry et ses idee a la con

So there you go, usually it's "coque d'onion farci de ...." The last comment refers to the fact that this was not the case for the menu of this restaurant, as is the case with yours - I suppose it's meant to sound posh ;o) and allows the chef to choose a filling according to whatever ingredients he has that week. In this case I imagine that there will be some kind of vegetable inside OTHER than the puree (which I am convinced remains outside of the onion) which would complete this kind of fish dish, such as peppers, courgettes and tomatoes...

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Note added at 5 hrs (2012-05-24 16:43:03 GMT)
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(excuse my extremely repetitive use of the word case in that last paragraph!)

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Note added at 5 hrs (2012-05-24 16:45:25 GMT)
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note: "stuffed onion" and not "onions" as there tends only to be one big one.

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Note added at 7 hrs (2012-05-24 18:16:45 GMT)
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Zap that last comment... In my experience they use a "big daddy" onion and it has the same name, indeed you did say onions which is why I did too :oD Good luck!