so.. i think i remember this class au (and similars) - made WITH something de (and similars) - made OF something so, if it was "sandwich DE poisson" it will mean that the actual bread was made out of fish. which is pretty unusual.. hope i helped and forgive my english mistakes. I'm brasilian

Here's how Inselstricken explained it on Yahoo:

one of the meanings of the preposition à is similar to 'with' you know that [ à + le ] is au, and [à + les ] is aux? yes, I think you realise that. So - à + a specified noun indicates a characteristic - which we can translate as 'with' à lait frais - the characteristic of the yogurt is that it is made with fresh milkj [ LAIT not lot!!] to get away from yogurt for a minute, you can say robe à manches courtes - the characteristic of the dress is that it has short sleeves, so in English you can say a dress with short sleeves - or in fact, a short-sleeved dress. Similarly you want to say yogurt with fruit - but look, yaourt aux fruits is actually, fruit yogurt - having fruit in it is a characteristic of this yogurt, it is not plain yogurt. If you were to say yaourt avec fruits, I would tend to assume that you had put some plain or vanilla yog in a dish, and some fruit compote alongside - something like a Müller Fruit Corner... café au lait is not coffee in the milk! 'milk'or 'milkiness'is a characteristic of the coffee - as you might say, milk coffee, a latte, not black coffee. Now think of the construction where we use à to show what something is FOR - you know that une brosse à dents is a toothbrush, not a brush with some teeth embedded in it! I'd like to suggest that you get yourself a really good grammar book, and dictionary, best you can afford, because words often have many many differnt meanings and can be used in many different ways.

[Link to source: https://answers.yahoo.com/question/index?qid=20121123005045AAKSryI ]

Sorry, I don't understand what you're asking.

All I can say is, the form "à de" is not possible (at least with this type of "à").

Just remember that this structure, whether with food or other items, is an equivalent of "made with / in the style of / in the flavour of" etc., as opposed to "made from, made for" or "accompanied with". And very often in English, this structure will take the form of a compound noun (= noun + noun):

• une glace au chocolat = a chocolate ice cream (chocolate is an ingredient) une glace avec du chocolat = an ice with some chocolate (on the side)

• des chaussures à talons = stilettos (=shoes made with high heels) des chaussures de sport = sports shoes (= shoes made for sports)

So, again, "un sandwich à de poisson" is a grammatical mistake ; "un sandwich de poisson", though grammatically corrct, makes no sense because it sounds like the sandwich (i.e. the bread) is made from fish.

Take "spelt bread" : in French it's "un pain d'épeautre", because the bread is made out of spelt (specific grain/cereal), and it's not some regular bread with spelt in between the slices ! ;-)

This is describing it as having a sandwich with fish on it, it's a characteristic of the sandwich (which is why you don't use avec here)

You should know by now that there's not much 'logic' to languages, despite what Duolingo seems to show sometimes.

"Au / à la" (remember "au" is just a contracted "à le") is the most common translation for "to the" and also "at/in the","au" before masculine nouns, "à la" with feminine, even "à l'" before a vowel:

• Je vais au bureau = I am going to the office

• Il est à la plage = he is at the beach

• Elle travaille bien à l'école = she works well in school

"Un sandwich au poulet", "des spaghetti à la carbonara" or "des sucettes à l'anis" is not about destination or location: look at it as in the style of maybe, but don't try to find any logic in it.