Sauté the onion and cabbage in a skillet with a small amount of oil over medium heat until softened, about 5–6 minutes.
Season with garlic powder, salt, and pepper.
In a bowl, combine sautéed mixture with corned beef and potatoes.
Lay an egg roll wrapper on a flat surface in a diamond shape.
Spoon 2–3 tablespoons of the filling near the center.
Fold the bottom corner over the filling, then fold in the sides and roll tightly.
Brush the top edge with the beaten egg to seal.
Repeat with remaining wrappers and filling.
Heat oil in a deep pan to 350°F.
Fry 3–4 egg rolls at a time, 3–4 minutes per side, until golden brown and crisp.
Remove and drain on paper towels.
Serve hot with your favorite dipping sauce.