Heat the vegetable oil in a wok or sauté pan. Add the garlic, ginger, cabbage, carrot, bean sprouts, shiitake mushrooms, celery and spring onions. Sauté for 3–4 minutes until softened and then add the soy sauce mixed with the cornflour. Remove from the heat and scrape into a bowl. Refrigerate until completely cool. Drain off any excess liquid.
On a clean surface, place two spring roll wrappers on top of one another (this helps make the shell extra crisp as the filling is slightly wet). It is easiest to position the wrappers with one corner facing towards you. Brush some of the egg white across the whole surface. Place about 3 tablespoons of the filling on the lower third of the wrapper. Bring the lower tip up and fold over the filling. Fold in the side flaps and roll tightly into a spring roll. It’s VERY important to keep the tension as you roll so it’s tight. Brush the finshed roll with egg white along the seal. The rolls can be refrigerated for up to 4 hours, layered between non-stick baking paper, and then wrapped in plastic wrap, or immediately frozen.
Heat the oil in a wok or deep medium saucepan until it reaches 180–190°C (350–375°F) or when a small piece of bread instantly sizzles. Fry five or six spring rolls at a time until crispy and drain on paper towels. Keep the cooked rolls in a warm oven.
Serve the spring rolls immediately with the mustard sauce.