Peel the potatoes, removing any bad spots, such as bruising and discoloration. Cut into big even-sized chunks. Place cut potatoes in water as you peel to prevent discoloration.
Wash the potatoes and rinse until the water runs clear. Place them in a large pot with a teaspoon or two of salt and enough water to cover them.
Bring to a boil, and cook until potatoes are fork tender. Drain the water from the potatoes, reserving about a cup of water.
Return the potatoes to the stove and simmer for about a minute to remove excess moisture.
Transfer potatoes into a large bowl and mash with a potato masher. Season with pepper, and let cool.
Combine spices in a small plate or bowl—allspice, mustard, nutmeg, cloves, and sage—in a small bowl and set aside.
Melt butter or oil in a large skillet over medium heat. As soon as the butter melts, add diced onions, garlic, celery, thyme, and bay leaves. Cook for 2-3 minutes.
Stir in the spice mixture and continue cooking for about a minute.
Add ground pork and beef, followed by one cup of reserved potato water.
Bring to a boil, reduce heat and continue simmering on low until meat is tender with a paste-like texture (this helps the pie filling hold its shape) and very little liquid is left in the skillet. Stir in beef bouillon powder.
Then stir in mashed potatoes, and cook for about a minute. Remove the saucepan from the heat and let the meat mixture cool to room temperature before proceeding to the next step.