1/4 cup soy sauce
2 tablespoons rice vinegar
1 tablespoon light-brown sugar
6 cups (48 ounces) vegetable oil
1 napa cabbage (about 2 1/2 pounds), thinly sliced
4 medium carrots, coarsely grated
4 garlic cloves, minced
1 tablespoon grated fresh ginger
coarse salt and ground pepper
1 pound ground pork
6 scallions, thinly sliced
16 egg-roll wrappers (6 to 7 inches square)
1 large egg, lightly beaten
bottled sweet-and-sour sauce and spicy mustard, for serving