Combine sugar, salt, white pepper, soy sauce, oyster sauce, and water in a small bowl to create the flavoring sauce.

Stir the flavoring sauce well and set aside.

Heat canola oil in a large skillet over medium-high heat.

Add chopped scallions and cook, stirring, for about 30 seconds until softened and aromatic.

Add shredded cabbage, chopped celery, and shredded carrot to the skillet, stirring to combine well.

Cook, stirring frequently, for about 1 minute, until the vegetables have collapsed slightly.

Add the prepared flavoring sauce to the vegetables, stirring to combine, and continue cooking for about 2 minutes, until most of the liquid has disappeared and the vegetables are just cooked through.

Add the char siu pork matchsticks to the vegetable mixture and continue cooking, stirring to combine the flavors and heat through, about 1 minute.

Give the cornstarch slurry (cornstarch dissolved in water) a final stir, then pour it over the filling mixture.

Cook for about 30 seconds, stirring constantly, to bind the mixture nicely.

Transfer the filling to a platter and spread it out in a thin layer to cool quickly.

Allow the filling to cool completely before using it to fill the spring rolls.

Line a baking sheet with parchment paper and lightly dust it with cornstarch to prevent sticking.

For each spring roll, place about 1/4 cup of filling slightly below the center of a spring roll skin.

Fold the bottom corner of the skin over the filling, then fold in the sides to create a tight roll.

Before rolling the spring roll up completely, brush the upper two edges with beaten egg to ensure a good seal.

Set the finished spring rolls, seam side up, on the prepared baking sheet.

Cover the spring rolls with a kitchen towel to prevent drying.

Heat 1 inch of canola or peanut oil in a wok, saucepan, or deep skillet over medium-high heat to about 350F (175C) (use a deep-fry thermometer to monitor).

Carefully slide a few spring rolls into the hot oil, being careful not to overcrowd the pan.

Fry the spring rolls for about 1 1/2 minutes, turning as needed, until they are light golden brown.

Remove the fried spring rolls from the oil and drain on paper towels.

Repeat the frying process with the remaining spring rolls.

After their first frying, refry the spring rolls for 45 to 60 seconds in 350F (175C) oil until they are crispy and golden brown.

Serve the hot spring rolls whole or cut in half diagonally, with the dipping sauce of your choice (sweet and sour sauce, rice vinegar, black vinegar, or balsamic vinegar with chile oil).