Place the ham in a large saucepan and cover with cold water. Bring to the boil, skim all the scum from the surface, and then reduce to a gentle simmer. Add the black treacle and 1 teaspoon of the thyme and simmer until the meat is just starting to fall off the bones, 2 to 3 hours.
Remove the meat from the liquid and let cool to room temperature. Strain the stock into a clean pan and return to a boil until reduced by one-third. Let cool (this will be the jelly for the pie later on).
Pick the meat from the ham hock, being careful to avoid the fat, and transfer into a bowl. Season with salt, pepper and freshly chopped thyme (hocks can be very salty so taste before salting, you may not need it). Remember this is a cold pie, so pepper will really bring out flavor once rested and chilled.
Heat the oil in a frying pan over medium heat. Cook the onions to sweat without any color, 5 minutes, and then let cool and transfer to a bowl. Stir in the cheese and cream, season with salt and pepper and mix well.
For the pie dough: Mix together the flour and salt in a bowl, and then make a well in the center. Combine the lard with 6 3/4 ounces water in a saucepan and bring to a boil, and then pour into the well of the flour mixture and stir with a wooden spoon to form a smooth dough. Let the dough cool 5 minutes if too hot to handle.
For assembling: Preheat the oven to 350 degrees F. Lightly grease a 3 to 4-inch deep, 6 to 8-inch diameter pie ring and line the bottom with a disc of lightly greased parchment paper. Place onto a parchment-lined greaseproof baking sheet.
Roll two-thirds of the dough out on a lightly-floured table into a circle large enough to line the pie ring and overlap the edge. Fit the dough into the pie ring, carefully pressing into the corners and allowing it to just hang over the edge. Roll the remaining pastry into a circle for the lid.
Cover the bottom of the pie with the ham meat, followed by the cheese mixture and then the soft-boiled eggs until the pie is filled. Sprinkle the egg layer with salt and pepper.
Brush the pie edges with the egg wash and place the pastry lid on top. Pinch the pie edges together with your thumb to crimp and create a seal, and then trim the overhang using a knife. Brush the top of the pie all over with more of the egg wash and make a hole in the middle of the lid.
Bake 1 hour, and then remove the pie ring and brush the sides and top again with egg wash. Baking 15 minutes longer, and then let cool to room temperature. Refrigerate 2 to 3 hours.
Check the pie for any holes and fill them with softened butter so that the jelly won't escape. Remove the top layer of fat from the ham jelly and slowly reheat until melted. Pour the jelly into the hole in the top of the pie until filled. Refrigerate the pie until the jelly is set, and then serve.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.