Begin with the marinade. In a large bowl, combine calamansi juice (katas ng kalamansi), soy sauce (toyo), thinly sliced onions (sibuyas), minced garlic (bawang), and cracked peppercorns (dinurog na paminta). Pat the pork shoulder dry and optionally tie with kitchen twine for even cooking. Place in the marinade, massage thoroughly, and refrigerate for 2-4 hours (hindi lalampas ng 4 na oras).
While the meat marinates, prepare the potatoes (patatas). Heat oil (mantika) in a large, heavy-bottomed pan and fry potato slices until golden brown. Remove and set aside.
Remove the pork from marinade, reserving the liquid and aromatics. Pat the meat dry and sear on all sides until well-browned in the same pan used for potatoes. Remove and set aside.
In the same pan, sauté the reserved onions and garlic from the marinade. Add the reserved marinade liquid, tomato sauce, water (tubig), and bay leaf (dahon ng laurel). Return the pork to the pan and bring to a simmer. Cover and cook on low heat for 2-2.5 hours until fork-tender.
Once the meat is tender, remove it from the pan. Stir liver spread into the sauce until well combined. Season with salt and pepper (asin at paminta) to taste. Let the meat rest for 15 minutes before slicing.
Arrange the sliced meat and fried potatoes on a serving platter, then pour the hot sauce over everything. Serve with steaming white rice (mainit na kanin) and garnish with fresh calamansi halves.