If necessary, allow 18 frozen lumpia or spring roll wrappers to thaw in the refrigerator until they can be easily separated, approximately 3 hours.
Trim the stems from 3 large portobello mushrooms and slice them into 1/2-inch-thick slices. Thinly shred 1/2 medium green cabbage until you have 8 cups. Trim 1 medium red bell pepper and cut it into matchsticks. Peel and cut 3 large carrots into matchsticks.
In a large frying pan, heat 3 tablespoons of olive oil over medium-high heat until shimmering. Add the mushrooms, cabbage, and 1/2 teaspoon kosher salt. Cook, stirring occasionally, until the cabbage wilts down by about half, around 5 to 6 minutes. Add bell peppers and carrots, cooking and stirring occasionally until the vegetables are crisp-tender, about 3 to 4 minutes. Incorporate 2 tablespoons soy sauce, 1 tablespoon Sriracha, 2 teaspoons toasted sesame oil, and 1/2 teaspoon ground white pepper. Toss to combine and continue to cook until there is no more moisture in the pan, approximately 10 minutes.
Transfer the mixture to a rimmed baking sheet, spreading it into an even layer. Allow it to cool completely for 30 to 45 minutes. Meanwhile, finely chop the white and green parts of 3 medium scallions.
Add the scallions to the vegetable mixture and toss to combine. (You should have about 4 packed cups of filling.) In a small bowl, combine 2 tablespoons water and 1 tablespoon cornstarch. Remove 18 spring roll wrappers from the package and place them on a work surface or plate, covering them with a damp paper towel to prevent drying out. Line a large baking sheet with parchment paper or a silicone baking mat.
Place 1 wrapper on the work surface in a diamond shape (keep the remaining wrappers covered with the towel). Place approximately 1/3 cup of the filling across the center of the wrapper, forming it into a 4-inch-long log. Fold the left and right sides of the wrapper tightly over the filling. Dip a finger into the cornstarch mixture and dab it onto the top corner of the wrapper. Roll from the bottom up into a tight log. Place on a cutting board or plate. Repeat the assembly process with the remaining spring rolls. If making ahead, you can freeze the filled spring rolls at this point.
Heat enough neutral oil to come up 1 to 1 1/2 inches up the sides of a large pot over medium-high heat until it reaches 370°F (about 5 to 6 cups). Meanwhile, fit a wire rack over a baking sheet.
Fry in batches of 3 to 4: Add the spring rolls to the hot oil and fry until golden-brown all over, turning with chopsticks or tongs as needed, for a total of 2 to 3 minutes. Transfer them to the wire rack. Serve with sweet chili sauce.
RECIPE NOTES:
- Wrappers: This recipe uses wheat-based spring roll wrappers. Some rice paper wrappers are labeled as “spring roll wrappers,” but they are not suitable for this recipe.
- Make-ahead: The filling can be prepared a few days in advance and refrigerated. The cooler the filling, the easier it will be to wrap the spring rolls.
- The spring rolls can be assembled up to 1 day ahead and refrigerated, wrapped tightly in plastic wrap, or frozen solid and then stored in a zip-top bag for up to 2 months. Fry frozen spring rolls directly from frozen for about the same amount of time.
- Storage: Refrigerate leftovers in an airtight container for up to 4 days. Reheat in the oven, air fryer, or toaster oven.
