In a large bowl, stir together cream cheese and Thousand Island until smooth.: The aroma of the Thousand Island dressing blending with the softened cream cheese is mildly sweet and tangy, and you should see a glossy, uniform mixture when it is ready. This base smooths the filling and helps everything adhere, which matters because it prevents loose bits from falling out during frying. A common mistake is using cold cream cheese , which will result in lumps; make sure it is soft to the point where a spoon glides through. If the mixture seems too thick, a teaspoon of extra dressing can loosen it slightly, but avoid adding moisture that could make the wrappers soggy later. While stirring, note the texture, it should be creamy and spreadable, not watery or grainy.
Stir in corned beef, swiss cheese, and sauerkraut.: When you fold in the chopped deli corned beef , shredded Swiss cheese , and squeezed sauerkraut , the bowl becomes a colorful, textured mixture with flecks of meat and strands of cheese. The cheese will start to cling to the other ingredients, helping bind the filling as it melts later, while the sauerkraut adds bright vinegar notes. I pay attention to the distribution, making sure each spoonful has a balance of all components. A typical error here is not draining the sauerkraut thoroughly, which can make the filling too wet; squeeze firmly to remove excess liquid. Taste a tiny bit to check seasoning and balance, remembering the corned beef brings saltiness so you generally do not need extra salt.
Place a damp paper towel over egg roll wrappers so they don't dry out. Working one at a time, lay one out so that it forms a diamond in front of you. Spoon about 3 tablespoons of filling on it.: As you open a wrapper, you will notice its fragile, slightly powdery surface; the damp towel prevents cracking. Placing the wrapper as a diamond gives the right geometry for folding and sealing. The filling mound should sit slightly off center toward the bottom third, which helps with a neat roll. Avoid overfilling, which will cause leaks and messy frying. Sensory cue: the filling should look compact but not mounded, and you should be able to see bits of deli corned beef and strands of Swiss cheese . If the wrapper starts to dry, re-cover immediately to avoid tearing during folding.
Fold up bottom and sides and roll it up, sealing the seam by rubbing some water along it and pressing it.: The act of folding transforms the shape from flat to cylindrical, and the small amount of water you apply acts as an adhesive for the wrapper. Pressing firmly along the seam ensures it stays sealed while frying, keeping that hot oil from seeping into the filling. I run my fingertip along the seam and then tuck the edges tightly to create tension in the wrapper; this tension yields a smooth, evenly blistered surface when fried. A common slip is using too much water, which leads to soggy edges, so be sparing and precise. If a wrapper tears, patch it with a small piece folded from another wrapper, but do not overdo it, as patched areas can open while cooking.
Heat about 1 1/2 to 2 inches of oil in a Dutch oven or deep skillet. Heat to 360 degrees.: When the oil reaches the right temperature you will see a slight shimmer and, if you use a thermometer, a steady 360 degrees. Proper heat is essential because oil that is too cool soaks into the wrapper, producing greasy results, while oil that is too hot browns the exterior before the interior heats through. I like to test with a small corner of wrapper; it should sizzle and brown steadily without burning. Use a stable, heavy pot like a Dutch oven to maintain temperature, and keep an eye because adding multiple egg rolls will drop the oil temperature. A frequent error is overcrowding the pot, which leads to uneven cooking, so fry in batches.
Fry 4 egg rolls at a time until golden on both sides, about 3 minutes for each batch. Place on a paper towel-lined plate. Serve with Thousand Island dressing for dipping.: As they fry, you will hear a lively, steady sizzle. Watch as the wrappers change from pale to rich golden brown, the surface becoming crisp and blistered; this visual cue tells you they are ready. Turning them so each side browns evenly gives a uniform crust and ensures the interior heats through and the cheese melts. After frying, let them rest briefly on paper towels to drain excess oil and to let the filling set, which improves handling. A common mistake is pulling them out too early, which results in under-melted cheese, or leaving them too long, which can dry the filling. Serve warm with extra Thousand Island dressing so guests can add more tang if desired.