Make the filling. Slice the carrot into long, thin lengths. Then cut into long, thin a€˜julienne’ sticks. Cut the leek into three, halve, then cut into sticks. Slice the celeriac and celery, then cut into julienne sticks as before.
Halve the onion lengthways, then slice thinly. Slice the peppers into sticks. Cut the chilli into thin shreds. Cut the ginger into small julienne sticks and chop the garlic. Mix everything together in a bowl with half the picked herb leaves.
Heat olive oil in a wok until very hot and add half the veg. Toss for 30 secs to wilt, then remove from the heat, season and toss in soy and sesame oil. Drain in a colander set over a bowl. Repeat with remaining veg. Mix in bean sprouts, sesame seeds, remaining herbs and extra soy, if liked.
When the filling has cooled, peel off a spring roll wrapper - it will feel slightly sticky. Cover the remainder with a tea towel. Lay the wrapper on a board and brush the edges with egg white. Spoon a sixth of the vegetables in a line on the top third of the wrapper.
Fold over the top of the wrapper and press well down all round. Fold in the sides, then roll up firmly, pressing the end in well. The roll should be about 18cm long. Repeat process with the remaining filling and wrappers. The rolls can be chilled at this point for up to 3 hrs, uncovered, in the fridge, until ready to cook. (You should only cook the rolls just before you’re ready to serve, so that they remain crisp.)
Heat a deep fat fryer or large pan a third full of oil to 180C (or until a cube of bread crisps and turns brown). Deep-fry two rolls at a time for 5 mins until golden and crisp. Drain on paper towel. Mix ingredients for the sauce and pour into 1 larger or six very small bowls. Trim the ends off the rolls, then cut in half diagonally. Serve on one large or six small plates with sauce. Garnish with herb leaves or salad.