Make the Filling

1. In a small bowl, combine the ground pork, soy sauce, and 1 teaspoon cornstarch. Mix well and marinate for at least 10 minutes.
2. Shred the cabbage and carrots using a food processor or by hand. Thinly slice the shiitake mushrooms or pulse them in a food processor until finely diced.
3. Heat a wok or large sauté pan over high heat. Add 1 tablespoon soybean oil and swirl to coat the surface.
4. Add the marinated pork and stir-fry until no longer pink, about 2–3 minutes.
5. Lower the heat to medium-low and push the pork to one side of the pan.
6. Add the garlic, cabbage, carrots, green onions, ginger, mushrooms, and water chestnuts to the empty side of the pan. Stir-fry for about 1 minute until vegetables are slightly softened.
7. Stir in the rice wine, additional soy sauce if needed, sugar, sesame oil, and black pepper. Continue stir-frying for another minute.
8. Strain filling using a colander over a mixing bowl to remove excess juices and then transfer the filling to a baking sheet and spread it out.
9. Allow the filling to cool for 15 minutes.

Prepare for Wrapping

1. Once the filling has cooled, discard any accumulated juices.
2. Blot the filling with paper towels to remove any remaining moisture or oil.
3. Keep the egg roll wrappers and assembled rolls covered with plastic wrap to prevent drying.
4. Use 1 heaping tablespoon of filling per wrapper to avoid overfilling and tearing.

Wrap the Egg Rolls

1. Place the filling toward the bottom third of the wrapper. Fold the bottom over the filling, then fold in the sides and roll tightly.
2. Brush the edge of the wrapper with the cornstarch-water mixture or beaten egg to seal.
3. Arrange wrapped egg rolls in a single layer. If stacking, separate layers with parchment paper to prevent sticking.

Fry the Egg Rolls

1. Fill a wok or deep pot with about 2 inches of high-heat oil.
2. Heat the oil to 350°F (175°C), or until a cube of bread turns golden within 10 seconds.
3. Gently slide 4 to 6 egg rolls into the oil at a time, frying and turning occasionally, until golden brown, about 1½ minutes.
4. Transfer to a wire rack to drain and cool slightly before serving.

Frying Frozen Egg Rolls

1. Fry egg rolls directly from frozen without thawing.
2. Add an extra 1½ minutes to the fry time, turning occasionally until golden and crisp.
Make the Sweet and Sour Sauce
3. In a medium saucepan, combine the pineapple juice, white sugar, brown sugar, rice wine vinegar, soy sauce, ketchup, red pepper flakes (if using), and cornstarch.
4. Cook over medium heat, whisking constantly, until the sauce thickens to the consistency of warm honey, about 3 to 5 minutes.
5. Serve the sauce warm or let it cool to room temperature.
6. To store, cover and refrigerate for up to one week.