For the dipping sauce: Whisk together the fish sauce, sugar, cilantro, basil, serrano chiles and lime juice in a small bowl until the sugar is dissolved. Set aside.
For the beef roll: Toss together the beef with the soy sauce, cornstarch, dried red chile flakes, white pepper, dark soy and sugar in a medium bowl.
Heat a wok or large nonstick skillet over high heat until very hot, about 1 minute. Add the peanut oil, then the beef. Cook, stirring or shaking the skillet occasionally, 1 to 2 minutes. Add the mirin and cook an additional 30 seconds to 1 minute. Transfer the beef to a plate to cool.
To fill the spring roll wrappers: Lay a wrapper on a cutting board. Arrange some beef strips across the middle of each wrapper; top with sliced scallions, serrano chiles and about 2 tablespoons mozzarella. Roll the wrapper over the filling, tucking in the ends after the first roll. Repeat with the remaining wrappers and filling ingredients.
Heat a heavy-bottomed pot over high heat and fill it to a quarter of its depth with peanut oil. Heat until a deep-fry thermometer inserted in the oil registers 350 degrees F.
Fry the spring rolls until golden brown and crispy, 2 to 3 minutes.
To serve: Slice the spring rolls in half on the bias and serve with the dipping sauce.
Recipe courtesy of Ching-He Huang