Sauté onion, carrots, celery & garlic in 2 teaspoons water for 5 min. Place in a heavy 3 quart pot along with remaining ingredients except 1/3 cup water & arrowroot.
Bring to a boil, cover partially, and simmer for 15-20 minutes until vegetables are tender.
Whisk together cold water & arrowroot until smooth & fully dissolved. Slowly pour into simmering sauce, stirring constantly.
Lower heat & continue stirring as sauce thickens and becomes shiny, about 5 minutes. If necessary, add more dissolved arrowroot or cornstarch, a teaspoon at a time, until sauce reaches desired consistency.