Why sauce au vin rouge ?

Question

I'm a bit confused as to how to modify a noun in some instances. For example, this exercise uses "sauce au vin rouge"  (red wine sauce), and marks "sauce vin rouge" incorrect while another exercise uses "sauce tomate (tomato sauce) and marks sauce à la tomate" incorrect. Can you explain the différent constructs and why one is correct and the other isn't?

Céline Pickard · Accepted Answer

Bonjour William,
Could you let me know in which KwizIQ exercise you used "sauce à la tomate" please? I would like to check this because, in essence, it is not ungrammatical. But, context is key etc. So "sauce à la tomate" is correct grammatically but it has become the "established" name for it, i.e. a set phrase.
Note that in French, there is a few of similar expressions actually: "sauce soja, sauce curry, sauce moutarde, sauce cocktail..."
I hope this is helpful.
Bonne journée !