In a small bowl combine the sherry, soy sauce, and salt and stir well.
To make the sweet and sour sauce: In a small bowl combine the water and cornstarch, and stir to mix them well. Set aside. Combine the chicken stock, vinegar, sugar, ketchup, and pineapple juice in a small saucepan. Stir with a fork or a whisk to combine everything well. Bring to a gentle boil over medium heat and cook 1 minute. Stir in the cornstarch mixture and cook just until the sauce becomes shiny and thickened. Remove from the heat and keep warm.
Set a serving platter by the stove, to hold the peppers and onions after their initial cooking, as well as the finished dish.
Heat a wok or a large, deep skillet over high heat. Add the oil and swirl to coat the pan. Add the ginger and garlic, toss well and cook until fragrant about 15 seconds. Add the bell peppers and onions and cook, tossing often, until fragrant and beginning to wilt. Add the pineapple and cook 1 minute more. Scoop the mixture onto the serving platter, leaving as much liquid behind as possible, and set aside.
Let the pan heat up again, and scatter in the pork. Spread it out into a single layer, and cook undisturbed until the edges change color, about 30 seconds. Toss well and cook, tossing often, until most of the pork is no longer pink.
Add the sherry-soy sauce mixture and toss well. Return the bell pepper mixture to the pan and toss to mix everything together well. Add the sweet-and-sour sauce and cook, tossing often, until all the ingredients are evenly seasoned. Add the green onion and toss again. Transfer to a serving platter and serve hot or warm.