Preheat a medium cast iron skillet with 1 1/2 - 2 cups of vegetable oil over medium heat. You'll need a good inch of oil in the pan. If you have a deep fryer, preheat to 350 degrees Fahrenheit.
Trim the kohlrabi of it's leaves and reserve for another use. I like to sauté the greens, and use them in frittatas, or warm grain salads.
Using a box grater, or a food processor fitted with the grater attachment, grate the kohlrabi and carrots. Add to a large bowl along with the cilantro, scallions, ginger, garlic, lemongrass, and thai chile.
Heat a large sauté pan or wok over high heat. Add in 2 tablespoons of vegetable oil, and carefully pour in the vegetable mixture.
Using a wooden spoon, continually turn the mixture over until the veggies have released some of their water, and have become slightly translucent, about 5-7 minutes.
Add in the soy sauce and salt, mixing thoroughly. Set aside to cool.
Get a cookie sheet and damp paper towels ready for keeping the rolls hydrated after you roll them. Mix 1 tablespoon corn starch with 1 tablespoon water, and mix well in a small bowl. Set aside.
Take one egg roll wrapper, with one pointy end facing you, and place 1/3 to 1/2 cup of the vegetable mixture on the bottom half of the egg roll.
Fold the bottom up and over the mixture, using the egg roll to pull the mixture tight, as if you were rolling a burrito.
Roll up slightly, and press down on both sides sides of the filling, sealing the mixture in.
Using your finger, dip in the cornstarch slurry, and moisten the edges of the egg roll. Fold in the sides, and secure. The slurry helps the wrapper stick to itself, so you can think of it as a glue.
Working fast so the slurry doesn't dry, finish the roll on the wrapper. Place on the cookie sheet under the damp paper towels while you continue with the remaining rolls.
Once you have all of the mixture in rolls, place them in the oil, 3 at a time, and fry on one side until golden brown, about 1 minute. Turn over and fry on the other side for a minute more. Reserve to a paper-towel lined plate until the rolls are all fried up. Serve with the sweet & sour sauce, and enjoy!