Freeze the pork until firm enough to slice easily, about 30 minutes. Slice across the grain into thick slices. Cut into 1-inch by 2-inch pieces. In a large bowl, combine the stock, soy sauce, vinegar, cornstarch and cinnamon. Add the pork and toss gently. Cover and let stand 20 minutes. In a large nonstick frying pan over medium-high heat, heat the oil for 2 minutes. Add the scallions, ginger and garlic; stir-fry for 10 seconds, or until the scallions are translucent. Add the pork and marinade. Stir-fry until the pork begins to brown, 2 to 3 minutes. Add the pears and stir-fry 1 minute. Add the spinach. Cook until the spinach begins to wilt, about 1 minute.
Ready in 1 hour