Heat a large cast-iron skillet over medium-high heat. Melt 2 tablespoons of the butter and add onions, salt and pepper and cook till they turn translucent, 5 to 7 minutes. Stir in the hash browns and spread in an even layer. Cook until crisp on the bottom, about 2 minutes. Add the remaining 1 tablespoon butter, season with salt and stir to combine. Spread them out and continue to cook, stirring occasionally, to get an even crisp on all the potatoes, about 7 minutes. Once cooked and crisped throughout, remove to a medium bowl.
Heat 1 inch of oil a high-sided skillet over medium-high heat until it registers 360 degrees F on a deep-fry thermometer. Line a baking sheet with a wire rack.
Slice the corned beef into short strips and add to the hash browns along with the sauerkraut and toss to combine.
Crack the egg into small bowl, add a splash of water and whisk with fork. Lay 1 egg roll wrapper with the short side closest to you on a clean, dry surface. With the tips of your pointer and middle fingers make a border of egg wash on the edges of your wrapper. Put 1/3 cup of the filling on the wrapper 1/3 of the way up from the bottom. Fold in the sides over your filling and wrap and tuck your egg roll together. Repeat with the remaining wrappers and filling. (You will have extra filling that you can use for omelets or breakfast or to make more egg rolls!)
With tongs, add 4 egg rolls seam side down carefully into the hot oil. Once the egg rolls are bubbly and lightly golden brown, flip and cook the other side until golden brown all over, about 3 minutes. Remove to the prepared baking sheet. Repeat with the remaining egg rolls.
Whisk together the mustard and honey in a small bowl. Put the amba sauce in another small bowl. Serve alongside of your egg rolls.
Amba sauce is a pickled, spiced mango condiment popular in Middle Eastern cooking.