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.