Cook ground beef over medium heat. Add chopped onion and cook together. Onion adds natural sweetness and depth. This is what makes the filling taste complete. Add garlic and cook until fragrant.
Add cabbage and carrots. Cook only 3–4 minutes. Do not overcook. You want the vegetables slightly softened, not watery.
Add soy sauce, salt, pepper, and sesame oil. Soy sauce adds salt and umami. Taste the filling before rolling. It should be slightly stronger than you expect because the wrapper will balance the flavor.
Let the mixture cool slightly before wrapping. Warm filling can tear the wrappers.
Spread the filling on a plate and let it cool. This step prevents soggy lumpia.
Lay one wrapper in a diamond shape. Place about 2 tablespoons of filling near the bottom corner.
Fold bottom corner over filling.
Fold both sides inward.
Roll upward tightly.
Seal the top edge with beaten egg.
Heat oil to 350°F (175°C).
Fry in small batches for 3–4 minutes until golden brown.
Remove and drain on paper towels.