I have in my kitchen a jar of Maille Dijon Original Prepared Mustard. What exactly does “prepared” mean in this context? What would “unprepared” mustard be?
Just off the top of my head, “prepared” means they’ve ground up the mustard seed for you and mixed it with vinegar. “Unprepared” mustard [here DDG heroically avoids making the inevitable Boy Scout joke] would be mustard seed au naturel.
The entry on Prepared Mustard, from the Cooking Encyclopedia.
So you need the modifier on the word mustard to distinguish it from its component ingredient.
Here in Britain, the best selling mustard (Coleman’s English Mustard) can be bought as a powder or ready-made. You just mix the powder with water to make it usable.
Much the same as that other yellow kitchen staple, custard.