Heat a large skillet or wok over medium-high heat. Add the oil, then sauté the onion, scallions, and garlic until fragrant, about 1 minute.

Add the cabbage, carrot, bell pepper, and water. Season with salt, soy sauce, and white pepper. Cook over medium heat for 3 to 4 minutes, stirring often, until the vegetables are tender and any liquid in the pan has completely evaporated.

Stir in the sesame seeds, then transfer the mixture to a plate and let it cool for at least 5 minutes before wrapping.

Lay a wrapper on a flat surface in a diamond orientation. Keep remaining wrappers covered with a damp cloth so they stay pliable. Place a portion of filling on the bottom corner of the diamond.

Fold the bottom corner over the filling, tucking it under to compress the vegetables. Roll halfway up, then dab the side corners with water and fold them toward the center. Finish rolling to the top point and seal with a final dab of water.

Heat enough oil in a large skillet to submerge the bottom half of the rolls over high heat—the oil is ready when it shimmers and releases small bubbles. Fry the rolls in batches for 2 minutes per side until deeply golden brown and crisp.

Drain on a wire rack or paper towels and serve hot with sweet chili sauce while the wrappers are still crackling.