Add half the canola oil to a large heavy skillet over medium-high heat. Cook the napa cabbage for 8-10 minutes, stirring until most of the liquid evaporates.
Include the garlic, carrots, and bamboo shoots. Cook for 1 minute, stirring consistently. Stir in the mirin, soy sauce, and sesame oil. Let the filling cool completely.
Mix the cornstarch with 2 tablespoons of water in a small bowl. This mix will help seal the spring rolls.
Lay out a spring roll wrapper. Brush the cornstarch slurry around the edges, place about 2 tablespoons of filling in a long rectangle shape. Fold in the sides, and roll the wrapper tightly.
Heat 3 inches of oil in a Dutch oven to 325 degrees Fahrenheit. Fry the spring rolls until they are golden brown, which should take about 2-3 minutes.
Notes
Ensure the cooked filling cools completely before rolling to avoid tearing the wrappers.
Keep the oil at a consistent 325°F for even frying and perfect crispness.
Take care while folding the sides of the wrappers to prevent any openings.
Fry the spring rolls in small batches to maintain oil temperature and avoid sogginess.
Taste the filling before rolling to adjust soy sauce or seasoning.