Boil the pork belly and pig's ears in a large pot of water until tender, about 1-1.5 hours. Drain and let cool.
Dice the boiled pork belly and pig's ears into small cubes.
Heat a large skillet or frying pan over medium-high heat. Add a small amount of oil.
Add the diced pork belly and pig's ears to the pan. Cook until they are crispy and browned, about 5-7 minutes.
Stir in the minced garlic and chopped onion. Cook until the onions are translucent and fragrant.
Add the liver spread or finely chopped chicken liver. Cook for an additional 3-4 minutes, mixing everything together.
Stir in the green chili peppers, soy sauce, vinegar, and calamansi juice (or lemon juice). Adjust the seasoning with salt, black pepper, and sugar if desired. Cook for another 2-3 minutes.
If using an egg, make a small well in the center of the sisig mixture and crack the egg into it. Cover and cook for a few minutes until the egg is cooked to your liking.
Garnish with chopped green onions and sliced chili peppers.
Enjoy your meal!