Methods:

Begin by gathering and prepping all of your ingredients (except for the puff pastry sheets—keep those frozen) according to the ingredients list above.
Place a large, deep skillet or Dutch oven oven medium-high heat, and add the butter and the olive oil. Once the butter is melted, add in the onion, carrot, celery, plus a couple of good pinches of salt and pepper, and saute the veggies for a few minutes until just softened.
Add in the garlic, and once aromatic, add in the ground beef along with the Italian seasoning, the white pepper plus a couple more pinches of salt and pepper, and use your spoon or spatula to crumble the beef into a fine consistency, cooking it for a few minutes until no longer pink.
Next, add in the tomato paste along with the Worcestershire sauce and stir that into the beef mixture, then sprinkle in the flour and stir that in to blend well.
Add in the beef stock or broth, followed by the diced potato, and stir to combine. Cover the pan and allow the filling to gently simmer just until the potatoes are tender and the sauce thickened, roughly 25-35 minutes.
To finish the filling, add the thyme leaves, the chopped parsley and the peas, and stir to combine. Check to see if any additional salt or pepper is needed, and allow the beef filling to completely cool.