Cut fillets into 2 x 3/4" strips.
Combine lemon juice and soy sauce. Add fish and let stand 20 minutes.
Add 2 tablespoons oil to wok or large skillet. Heat over medium high heat for 2 minutes.
Add red pepper stir fry 2 minutes. Add celery, snow peas and mushrooms stir fry 2 minutes more.
Remove vegetables and set aside.
Add remaining tablespoon oil to wok and heat over medium high heat for 2 minutes.
Drain fish reserving marinade. Add fish to wok and stir fry 2 minutes or until fish flakes easily.
Return vegetables to wok.
Combine cornstarch, water, salt, pepper and reserved marinade stir until smooth.
Add to wok and cook 2 minutes or until slightly thickened. Serve over hot cooked rice.