Meanwhile, in a small bowl or measuring cup, whisk together the soy sauce, vinegar and sugar.
Slice the pork into 1/2-inch-thick rounds, then slice again to make 1/2-inch-wide strips. Place the pork in a medium bowl and sprinkle with the cornstarch and salt and pepper, and toss to coat.
In a large skillet or wok, heat the oil over high heat until very hot. Add the pork and stir-fry until the pork is almost cooked through and begins to brown, about 2 minutes. Add the garlic and bell peppers and stir-fry until the vegetables begin to soften, about 5 minutes. Add the pineapple and scallions and continue to stir-fry until the pork is cooked through and the vegetables are tender, 1 to 2 minutes longer.
Per serving (does not include optional peanuts): Calories 444; Total Fat 8 grams; Saturated Fat 2 grams; Protein 35 grams; Total Carbohydrate 56 grams; Sugar: 15 grams; Fiber 4 grams; Cholesterol 92 milligrams; Sodium 737 milligrams;