In a dutch oven or large heavy skillet, heat 1 tbsp oil over medium high heat; cook onions, red pepper, celery, diced potatoes, garlic, salt, thyme, cinnamon, nutmeg, allspice, pepper and cloves stirring occasionally, for 5 minutes or until softened. Add mushrooms and cook until most of the moisture has evaporated
Add pork, breaking up with a wooden spoon, and cook until no longer pink. Drain off fat from pan.
Add 3 cups stock, and simmer over medium heat for about an hour, until broth is reduced, potatoes are extremely tender, and a thick sauce has started to develop. Low and slow is the trick here, I like to simmer for 1 1/2-2 hours on super low heat. You may need to add additional broth up to 5 cups depending on how large you cut the potatoes and how long you simmer the mixture.
Taste and adjust seasoning as needed.
Fill bottom of a pie shell to the top with meat filling, put top on and cut 2 steam vents.
Bake at 350 for 30 minutes or until golden brown and bubbly.
If you want to make the filling ahead of time, it freezes really well. Place about 6 cups of filling into a freezer safe container and simply defrost and use as needed.