Start by preparing your wrapper batter. In a large bowl, whisk together cornstarch and water until smooth. Add the beaten eggs, salt, sugar, and oil, mixing until you have a thin, runny batter. Let this rest for 15 minutes while you prepare other ingredients.
Begin making your filling. Heat oil in a wide pan over medium heat (350°F/175°C). Add chopped onions and minced garlic, cooking until soft and fragrant, about 2-3 minutes. Add the diced pork belly and cook until it starts to brown, around 5-7 minutes. Add the chopped shrimp and cook until they turn pink, about 2-3 minutes. Pour in the fish sauce and cook for another minute.
Add half a cup of water to the pan and let the pork simmer until tender, about 10-12 minutes. Add your julienned palm heart (ubod), lower the heat to medium-low (300°F/150°C), cover the pan, and cook for 10-15 minutes until the palm heart becomes tender. Season with salt and pepper to taste. Drain the filling well and set aside to cool slightly.
While the filling cools, make your peanut sauce. In a saucepot over medium heat, combine water, soy sauce, brown sugar, and salt. Bring this to a boil, stirring until the sugar dissolves completely. Add minced garlic and ground peanuts, cooking for 2-3 minutes. Mix cornstarch with a quarter cup of water until smooth, then whisk this mixture into your simmering sauce until it thickens. Stir in the peanut butter until well combined. Keep your sauce warm over very low heat.
Now make your wrappers. Heat a 9-inch non-stick pan over low heat (250°F/120°C). Spray the pan with cooking spray. Pour a quarter cup of your rested batter into the center of the pan, quickly tilting the pan in all directions to spread the batter evenly. Cook until the edges start to pull away from the pan and small bubbles form in the middle, about 30-40 seconds. Gently flip the wrapper and cook for another 10 seconds. Slide the finished wrapper onto a plate. Repeat this process, spraying the pan between each wrapper, until you've used all your batter.
To assemble your lumpia, lay a wrapper on a clean surface. Place a fresh lettuce leaf on top. Spoon about a quarter cup of your cooled filling across the middle of the wrapper. Fold the sides over to cover the filling, similar to wrapping a burrito.
Serve your lumpia immediately by placing it on a plate, generously pouring the warm peanut sauce over top, and sprinkling with extra ground peanuts and minced garlic.
Remember to keep your wrappers covered with a slightly damp cloth while working to prevent them from drying out, and don't overfill your lumpia or the wrapper might tear. If your sauce becomes too thick while working, simply stir in a little warm water to reach the right consistency.