This sauce is redacted from The Opera of Barolomeo Scappi. The original recipe called for grape must, but as I do not live in wine country, I was not able to find any in my area. All research I did said a good substitution was to simmer grape juice until it was 1/3 it’s previous volume.
Start with 1 cup of grape juice, and simmer it until there is only 1/3 cup left. To the Must substitute, add 2/3 cup of roast juices, 1 teaspoon apple cider vinegar, 1/2 teaspoon tapioca starch, 1/4 teaspoon cinnamon, 1/4 teaspoon nutmeg, 1/4 teaspoon ginger, dash of cloves, 1/4 teaspoon garlic powder, salt and pepper to taste. Bring to a simmer, and let simmer for 5 minutes, until slightly thickened. Keep warm until serving.
This is a good sauce for either beef or pork roast.