Saute the garlic in the oil in a medium saucepan for 2-3 minutes.
Add the remaining ingredients except for the cornstarch and water. Stir and bring to a simmer and let simmer for 20 minutes, stirring occasionally.
Stir together the cornstarch and water. Slowly whisk into the sauce. Bring to a slow boil while still stirring, then remove from heat. Sauce should be nice and thick but not gloppy. Add a bit of water if it's too thick.