Heat butter in a pan over medium-high heat. Add shallots and cook until soft, about 10 minutes. Add garlic, port and thyme and stir until reduced to a syrup. Transfer to a bowl over ice to cool.
Grind sea salt and peppercorns, and add in the allspice and freshly grated nutmeg. Add the mixture to the cooled pan of shallots and whisk in cream and eggs.
Pulse Mangalica liver in a food processor until finely chopped, and add to the shallot mixture along with the minced pork shoulder. Mix well using your hands or a wooden spoon.
Line the bottom and long sides of a terrine mold crosswise with about 6-9 strips of pancetta, arranging them close together (but not overlapping) and leaving a 1/2- to 2-inch overhang. Fill terrine evenly with ground-meat mixture, tapping terrine on the kitchen bench to compact it (it will mound slightly above the edge).
Cover the top lengthwise with 2-3 more pancetta slices to cover the terrine completely. Place the three bay leaves over the top and fold back the overhanging ends of the pancetta. Cover terrine with cling film and chill for at least 8 hours to marinate the meats.
Discard the plastic wrap and cover the terrine tightly with a double layer of foil.
Bake terrine in a water bath in a pre-heated oven at 160°F for 1.5-2 hours until the thermometer inserted diagonally through foil, at least 2 inches into the centre of the terrine registers 155 to 160°F. Remove foil and let the terrine stand in the mould on a rack for about 30 minutes.
Put the terrine in its mould in a clean baking pan. Put a piece of wax paper over the top of the terrine, then place a heavy chopping board on top. Place a few heavy cans on top to compress the cooked terrine. Chill the terrine in the pan with weights until completely cold, at least 4 hours. Continue to chill the terrine, with or without weights, for at least 24 hours to allow flavours to develop.
Place terrine mould in warm water to loosen the edges, then turn it out onto a clean cutting board. Cut into ½ inch slices and serve on a plate with pickles and crusty bread.
Chef’s Tip: Mangalica pigs are woolly pigs from Hungary. This can be substituted with any good quality pork with a high fat content. Mangalica liver can be substituted with chicken liver. The pancetta needs to be heavily smoked.