Combine the sage, shallots, garlic, salt, pepper, and port in a nonreactive saucepan over high heat. Bring to a boil, lower the heat, and simmer for 5 minutes.
Add the stock, bring to a boil, and cook for 15 minutes.
Meanwhile, combine the cornstarch with the water. Stir the mixture into the sauce, simmer for 2 minutes, and remove from the heat. Serve immediately or store, refrigerated, in an airtight container for up to 2 days. Reheat in a saucepan over low heat.
Yield: 2 cups