Season bangus with salt and pepper. Bake at 180 degrees Celsius for 15 – 20 minutes. Shred and flake meat. Set aside.
In a pan over medium heat, saute the onions, garlic, celery, and carrots until softened. Season with salt and pepper. Add the bangus flakes and celery. Adjust seasoning with salt and pepper as needed. Transfer to a bowl and let cool. Add singkamas and egg to the bowl of cooled bangus mixture and mix until well combined and mixture sticks together.
Cut the lumpia wrapper into 4 small squares.
Place bout 1-2 tablespoons of filling into the bottom center of each lumpia wrapper. Form filling to a cylinder and roll up the lumpia tightly. Seal the edges with egg yolk mixture.
In a deep pan over medium heat, heat up the oil and deep fry the lumpia until golden and crispy. Drain excess oil and serve.