How To make Sisig
2 cups chopped celery leaves
1 1/2 cups finely chopped shallots
1 cup thinly sliced scallion greens
1/2 cup sherry vinegar
1/4 cup calamansi, yuzu or fresh lemon juice (see Note)
1/4 cup minced cilantro
3 serrano chiles, minced
1 garlic clove, minced
Freshly ground pepper
2 tablespoons peanut oil
Make the Marinade
In a medium bowl, combine the orange juice with the garlic, ginger, lemongrass, oil, chili paste, salt, pepper and five-spice powder.
Prepare the Pork Belly
Add the pork belly to the marinade and turn to coat. Transfer the pork and marinade to a sturdy resealable plastic bag and refrigerate for 24 hours, turning it once or twice.
Preheat the oven to 350°. In a small roasting pan, spread the carrots, celery, leek, cilantro, bay leaves and orange zest in a single layer. Remove the pork belly from the marinade and set it on the vegetables. Pour the pineapple juice over the pork, cover the pan tightly with foil and braise in the oven for about 3 hours, or until the pork is very tender. Remove from the oven, uncover and let rest for 1 hour. Discard the vegetables or reserve for another use.
Increase the oven temperature to 450°. Transfer the pork belly to a rack set over a rimmed baking sheet, fat side up. Season it generously with salt. Roast in the upper third of the oven for 15 to 20 minutes, or until browned and crisp. Remove the pan from the oven and let rest for 30 minutes.
Make the Sisig
Meanwhile, in a large bowl, combine the celery leaves with the shallots, scallions, vinegar, calamansi juice, cilantro, chiles, garlic, 1 1/2 teaspoons of salt and 2 teaspoons of pepper and toss well. Let marinate at room temperature for at least 1 hour.
Using a cleaver or other heavy knife, finely chop the pork. Heat the oil in a large nonstick skillet over high heat until very hot. Add the chopped pork in a single layer and cook until crisp and brown on the bottom, then stir once and cook, without stirring, until browned, 8 to 10 minutes total. Stir the pork into the sisig mixture and season with salt. Transfer to a platter and serve immediately.