Remove the grapes from the bunches and crush them thoroughly by hand in a very large bowl until they release all their juice.
Cover the liquid with a clean cloth and let it rest for about 36–48 hours, until you notice that fermentation begins (small bubbles will appear).
At this point, strain the mixture: place the pulp in a clean kitchen cloth, squeeze it well, and collect the extracted juice in a saucepan.
Then cook the fermented juice over low heat until it reaches a thick, jam-like consistency (this will take about 2 hours).
Before the grape must becomes completely dense, take a glass of the hot liquid and dissolve the mustard powder in it, stirring carefully until you obtain a smooth, creamy mixture.
Once the mustard is fully blended, add it back to the cooked must and mix thoroughly.
Serve cold, or store in sterilized, airtight jars for later use.