Add diced onion, sun-dried tomatoes, corn kernels, cilantro, avocado and salt to a large bowl. Gently toss ingredients with a spoon to combine.
Heat oil in a large, deep skillet over medium-high heat until hot but not smoking. You want enough oil to cover the egg rolls halfway.
Lay an egg roll wrapper on a cutting board with the corner facing you and brush egg onto edges of egg roll wrapper. Spoon about a tablespoon and a half of avocado mixture onto wrapper closer to the bottom.
Tuck the outer edges of the wrapper in over filling, then bottom edge and roll wrapper to encase filling. Repeat this process until all filling has been used. You should end up with around 10 egg rolls.
Working in two batches, use tongs to place egg rolls in hot oil, sealed side down. Cook for about 15-20 seconds, or until golden and crispy. Cook for another 15-20 seconds on the other side then use tongs to place egg rolls on a paper towel-lined plate.
Repeat this process with the second batch of egg rolls and set egg rolls aside while you prepare the avocado ranch dipping sauce.
Transfer the flesh of half an avocado to a small bowl and mash with a fork. Add ranch dressing and whisk ingredients to combine. Your sauce should have a thick, creamy consistency. Some small chunks of avocado will remain.
Transfer dipping sauce to a small bowl and place in the center of a large plate. Slice avocados in half on the diagonal and arrange around the dipping sauce cup. Enjoy!