After coating a large wok or heavy skillet with cooking spray, heat at medium high for about 2 minutes. Add the pepper, green onions, and garlic and stir fry for about 15 seconds before adding the celery, water chestnuts, snow peas, and mushrooms. Continue to stir-fry for 1 minutes, or until the vegetables are tender and crisp.
After combining the cornstarch and the next four ingredients, add them to the wok. Stirring frequently, continue to cook the pan over high heat until the sauce is slightly thickened. Add the tuna, and stir to coat well.