Finely shred the cabbage, julienne the carrot, thinly slice the bell pepper, and chop the spring onion, then mince the garlic and ginger.

Heat the vegetable oil in a wok over high heat, add the garlic and ginger and stir-fry for 30 seconds until fragrant, then add the cabbage, carrot, and bell pepper and stir-fry until just softened but still crisp.

Stir in the spring onion, soy sauce, vinegar, black pepper, and salt, toss everything together for a minute, then turn off the heat and let the filling cool completely to room temperature.

Lay a spring roll wrapper in a diamond orientation, place two tablespoons of cooled filling near the bottom corner, fold the bottom corner over the filling, fold in the sides, and roll tightly, sealing the final edge with the flour-water paste.

Arrange the assembled rolls seam-side down on a tray and let them rest so the flour paste seals firmly before frying.

Pour the deep-frying oil into a deep-fryer or heavy pot and heat it until it reaches a shimmering 175°C, hot enough that a small piece of wrapper dropped in sizzles and rises immediately.

Fry the rolls in small batches, turning occasionally, until the wrappers are deep golden brown and crisp all over.

Lift the fried rolls out with a slotted spoon and drain them on a wire rack or paper towels to remove excess oil.

Whisk together the chili sauce and tomato ketchup in a small bowl until smooth.

Let the rolls sit for a few minutes so the crust firms up and the filling inside settles, then serve hot with the dipping sauce.