1. After boiling, grill or pan-fry the pig's face parts and pork belly until crispy and slightly charred. Let cool slightly. 2. Once cooled, finely chop the grilled pork into small, uniform pieces. You can also use a food processor for a finer chop, but be careful not to over-process into a paste. 3. In a large mixing bowl, combine the chopped pork with the finely chopped red onion, minced garlic, calamansi juice, soy sauce, vinegar, black pepper, and chopped siling labuyo (if using). Mix well to ensure all ingredients are evenly distributed. 4. If desired, add mayonnaise to the mixture and stir until it creates a slightly creamy texture. 5. Heat cooking oil in a skillet or, ideally, a sizzling plate over medium-high heat until it's very hot. 6. Add the sisig mixture to the hot skillet or sizzling plate and cook, stirring occasionally, for 5-7 minutes until heated through and slightly caramelized. The goal is to get some crispy bits without overcooking. 7. If using, crack a raw egg directly onto the hot sisig just before serving and allow it to cook from the residual heat. Stir the egg into the sisig as you eat, if preferred. 8. Serve immediately while still sizzling hot, typically with a side of steamed white rice or as a standalone pulutan.