Heat canola and sesame oils in a very large skillet or wok. Add the onion and celery and sauté over medium-high, covered, for 4-5 minutes, stirring occasionally. Layer the bell pepper, mushrooms, and sprouts over the onion-celery mixture. Cover and cook for about 5 minutes, or until the sprouts are slightly wilted, then stir all the vegetables together. Dissolve the cornstarch in the stock or water, then stir in the soy sauce. Pour the cornstarch mixture over the vegetables in the wok, followed by the tofu and scallions. Stir together cook for 5 minutes more over moderate heat until the sauce has thickened up.
Garnish with the cashews and serve over brown rice.