Mix the syrup, vinegar, and beer to create a marinade.
Salt the pork and rub with freshly ground pepper. Heat the fat in a roasting pan over medium-high heat and brown the pork for 8-10 minutes on all sides. Pour the marinade into the pan to deglaze, then add the bay leaves, 2 onions coarsely chopped, dried prunes, raisins, pepper-grains, and allspice-grains.
Cover the pot and braise in the oven at 250°C (482°F) for about 2 hours, until the pork reaches an internal temperature of 63°C (145°F).
Mix the cornstarch with a little of the braising liquid to create a slurry, then stir it into the sauce over medium heat for 2-3 minutes until thickened.