Start by preparing all your vegetables. Chop the onions and garlic finely. Cut your carrots into thin strips. Dice the sweet potatoes into small cubes. Cut the green beans into diagonal slices. Wash the bean sprouts and lettuce leaves, then set aside.
Make your crepe wrappers first. Mix cornstarch and water in a bowl until smooth. Add beaten eggs, oil, salt, and sugar. Mix well to make a thin batter. Heat a 9-inch non-stick pan over low heat. Pour ¼ cup batter and quickly swirl to cover the pan. Cook until edges start lifting (about 30-40 seconds). Flip gently and cook another 10 seconds. Place on a plate and put wax paper between each crepe. Cover with a slightly damp cloth while you make the rest.
Now cook your filling. Heat oil in a large pan over medium heat. Cook onions and garlic until soft and fragrant. Add fish sauce and stir for 2 minutes. Put in your carrots, sweet potatoes, and green beans. Lower the heat, cover, and cook for about 7-10 minutes until tender but still crisp. Add bean sprouts and cook for 2-3 more minutes. Add sugar and season with salt and pepper. Drain the vegetables well in a colander.
While the filling cools, make your peanut sauce. Mix water, soy sauce, brown sugar, and salt in a pot. Bring to a boil, stirring until sugar dissolves. Add minced garlic and ground peanuts. In a small bowl, mix cornstarch with some water until smooth. Slowly pour this into your boiling sauce while stirring. Keep stirring until it thickens. Add peanut butter and mix well.
To put everything together, lay a crepe wrapper on a clean surface. Place a lettuce leaf on top. Put about ½ cup of filling across the middle. Fold the sides over the filling, then roll it up carefully. The wrapper is delicate, so handle it gently.
Place your finished lumpia on a plate. Pour the warm peanut sauce over it generously. Top with extra ground peanuts and crispy garlic. Serve right away while the sauce is warm.
Keep any leftover filling and sauce in separate containers in the refrigerator. If your sauce gets too thick later, just add a little warm water and stir well.