On a clean cutting board, remove the tentacles from the bodies of the squid and reserve. Make sure the tubes are cleaned out and split them open lengthwise so they look like a flat triangle. Using a matte knife, lightly score the squid in a crosshatch pattern. Cut each tube into four pieces of roughly equal size.
In a medium bowl bowl, combine the soy sauce with 1 teaspoon of the cornstarch and toss the squid in it to coat. Marinate while preparing for the rest of the dish.
In a small bowl, combine the water, miso, and balsamic vinegar with the remaining teaspoon cornstarch.
In a hot pan or wok, add the sesame oil and swirl to coat the pan. Add the squid and cook for 30 to 40 seconds. It will curl up naturally and brown. Working quickly, add the garlic, ginger, and chiles. Allow them to fry for 10 to 15 seconds stirring constantly (they can burn fast). Add the onion, mushrooms, and bell pepper and saute for another minute. Pour in the stock mixture and simmer until the sauce begins to thicken. Season with white pepper and additional soy sauce, if necessary. Serve over rice.