Combine all of the ingredients in a small bowl or jar, whisking in the cornstarch last. The sauce may be covered and refrigerated for approximately two weeks. The cornstarch will settle, so just stir well before using.
To make a stir fry: when ready to use, stir-fry approximately 4 cups of your favorite veggies over medium to medium-high heat in about 2 teaspoons of oil in a wok or 12- to 14-inch nonstick skillet. You’ll want to slice the vegetables into bite-size pieces and start with the veggies that take longest to cook. (I like to cook mushrooms, carrots, and broccoli a little longer than asparagus, snap peas, and bell pepper. When in doubt, cook separately and remove to a plate when cooked to your liking.)
Once the veggies are done, remove them all to a plate, add 2 more teaspoons of oil, and stir fry one pound of protein—shrimp, chopped chicken, sliced steak, tofu, etc.—until just barely cooked. Return the veggies to the skillet, and give the sauce a quick stir before pouring ½ to ⅔ of the sauce over top. Stir to incorporate, and continue to cook another minute or two to thicken the sauce. Sprinkle with sliced green onions, sesame seeds, chopped peanuts, cashews, etc., and serve over rice if desired.